Sorry, long week and even longer day/night... I just re-read your original post. Have you ever been able to see what your temps are? That can potentially identify/rule out thermal issues as the cause. Also, double-check all connections and all while your in your case but, if you see a fair bit of dust, especially if enough to obscure the path of airflow by 1/4, or even worse - more... then you need with zero doubt, be it the main cause or not, to clean the dust out of your case and especially heatsinks. If they are aluminum, then this is by far much more important as it\ll dissipate heat less efficiently and slower as well. Before doing so, get a tube of thermal paste ready as you'll need a small bit as well as some isopropyl (rubbing) alcohol as it'll be needed to clean off the old paste and get the surfaces clean on CPU and heatsink, as well as anywhere that has gunk. If you smoke, then you might want to, as the last step after dusting and rinsing off the heatsink with clean water, rinse it off with alcohol too. This will clean it better and ensure less dust sticks to it over time, especially at first. Just be sure you follow proper torque sequence, for sure at the beginning of taking off the heatsink, and definitely the entire time while putting on the heatsink back in the PC. Example, for a 4 screw item, start with screw 1 out of four if you label them going around in a circle, tighten it until the screws is one or two threads on, then the same for screw 3, then 2, then 4. Then repeat the process until the slightest resistance, or less, equally, going in that pattern, continuing on little by little until all screws are 100 % secure, but not over tightened. If you have any questions, feel free to ask.

Given the age of this unit power supply is only 1 issue.  Thermal paste and cpu fan is the other.

Recommended replacement is EVGA 700BR.

B2 BQ W1 N1 versions from the same vendor do not work.

https://www.bestbuy.com/site/evga-700w-atx12v-eps12v-80-plus-bronze-power-supply-dc-dc-technology-black/6346160.p?skuId=6346160

no CPU or GPU  temperatures stated.,  50C is par,  80 to 90C the CPU slows then at 90C the CPU self shuts off.

that magic of intel inside.

I guess it  only fails gaming, so heat is and issue, no"?    but no temps  even told.

why is cleaning lint packed up heat sinks hard, (as any shop on earth to do that , it is like SOP first thing DONE)

If it shuts down with many different OS then it is not he OS,

run smart tests on all HDD first, if not the OS will be corrupted.

I call overheated, until you use real posted temps, not "its ok" i need the numbers.

as speedstep said

overheating, or bad PSU.

you said it self shuts off, right,l that means all FANS dead. right?????????????

the full list is here,

1:overheating 90C is DOOM , new GPU cards do same now , what  card is yours?>

2: bad PSU

3: PSU sees, overloads on power rails and all PSU shut off, even with no HDD in the PC. at all.

4: overheating south bridge (it has the migical, S0 to s5 power controls inside)

5: shorts on the MOBO< Strip the PC and find when it starts working again.

6: bad power switch , wires frayed and short.

7: HS packed in lint, or the CPU grease is old hard and now cracked, (up grade to Shin Estu , TIM)

8: weak fans, or #7 weak from lint or bearings bad.

I responded guys I am just waiting for the mods to put my reply back. Apparently these forums are jacked up and if you edit your own reply for corrections more then once to fast they mark it as spam. That makes absolutely no type of sense. I'll wait one more day and see if they fix it. I notified the mods once already. Today makes twice. If they don't fix it I guess I'll have to retype everything out best I can from memory. Sorry about this. Thanks for all the help everyone.

Correcting posts does not mark them as spam.

Putting malware XSS links in or other violations of TOS would be moderated.

The 755 is over 12 years old.

This means you can get a replacement tower easily for $100 or less.

https://www.walmart.com/c/kp/dell-optiplex-755

maybe you need a bois update mines usto do the same while gaming and after a bios update is not doing it no more

That's interesting.... brings to mind thoughts of a correction in thermal controls, but I could be wrong. I would say, if you are absolutely certain it won't shut off during a bios update, try it, but if not, don't risk making your PC a brick. I still think thermals are the main candidate for being the issue though. 

Speedstep I didn't link any malware mate. This was an automated process. So yes I am pretty sure if you edit to fast it is going to get removed. I linked one Discord image of my temperatures and one csv file showing a Open Hardware Monitor report. The exact message I got on the link is this.

This reply was marked as spam and has been removed. If you believe this is an error, submit an abuse report.

If a mod manually touched my post I would assume they'd have common sense and do like most mods on any forums ran professionally and merely just edit the post to simply remove the links.I mean why would I post virus or malware links when trying to get help? I would think that would get me banned. You been around a lot longer and would know better then me, but that would be my thought process for it.

In either case I filed a report a total of 3 times including just now. I haven't so much as a single response even explaining why it was removed. So I am just going to retype everything because it doesn't seem like the mods are going to do anything any way. Before I do are those two links I mentioned going to be acceptable?

I am asking because I don't want to post them for you all if they are going to be a problem. Thanks in advance.

Also Thanks for the link Speedstep, but those look like duel cores. My rig is a quad core rig. Also I'll go on and say to answer some of the other questions asked. My card is a AMD Radeon HD 5450 1 gig card. Pretty sure it's the card that came with the system since it doesn't have it's own fan. I could be wrong about that. I'd rather fix this rig considering it was a gift from a friend so there is some sentimental value to me behind it.

I have 7 gigs of ram in it currently..I know it's not a ram issue because I recently tested and replaced half the ram in the machine. So I ruled that out.

My temperatures I'll type out until I know if I can post those links or not. I'll type them as they are read or appear from my sensor monitor in Ubuntu.

Core 0 49

Core 1 38

Core 2 44

Core 3 44

Temp. 52.5

Hard Drive 37

Average 44.1

Maximum 52.5

These are my main temperatures at idle. Maximum usually hits mid 60's when I run Second Life to test it and powers out around there. On Windows 10 late 60's to early 70's for both Second Life and Fortnite. I am assuming Maximum is my GPU since my GPU temp in Windows read about the same temperature. 

I've also had when in Windows 10 it power off on me for merely just opening a window while updating Fortnite. So I was at idle temperatures when that happened. I also noticed I was more prone to power outages when my external drives were hooked up then I was when they were not. For example when I was booted up in Hiren's Windows 10 preloaded environment I did a stress test on the card. When the drives were connected it powered off. When they weren't it completed the test.

Only reason I didn't think about the bios which is a good suggestion btw is because realistically the pc ran flawlessly with the same bios for years. So why would it be an issue now? That was basically my thought process behind that.

Anyway I think that was a lot of what I put in my original reply. If I remember anything else I'll add it later. Thanks for the help everyone.

There may be different reasons why this happens. Overheating may also makes the system to turn off if the temperature reaches a set threshold. If it is a problem with the SMPS, you may find it cheaper at

google discord virus and read.

"It is an infection that is distributed via the VoIP software program Discord."

that CSV file, most (not me) have no way to read CSV (excel) why not just use text. text is always SAFE..

examples.....

my CPU temps hit 80c when doing x.

my GPU temps hit 80c or more doing Z

all anyone needs are the max HIT temperatures and telling how that is done.  Passmark 3d or furmark or COD 10,

photos, put the photos in a folder, even call it \temp.

then click the camara ICON above and post it,  takes me 10seconds work to post any photos of any kind.

well not 20feet wide, sure. (off my Canon EOS)

do not use Discord (please) if it's what I think it is... IDK for sure,  www is huge and lots of names similar.

or buy used 3020 for $50

there are far better and newer Dells sold used for near pocket change.  buy one. is best.

core i3 to i7, gen5 up is best. (MS  RULES ! Win 10 64bit)

oh, my bad,  it's and  heirloom PC.  got it.  ( but we now know it is HARDWARE failing, or PSU power issues)

even the ageless , "the 1 more straw that broke the camel's back."

PSU  290watts, and you pulling (using) 280watts gaming and you add 2nd HDD that uses 25more watts and BAM the camel's nose hits the sand and dies.

but no statement by you what you own, what do you have IN THE WHOLE, not stated. yet.

bad PSU

or just you  overloading the poor week thing, MT PC 755, got this, still.????????

minitower, MT PSU

so did it shut down in UBUNTU, I guess NO.

I guess only gaming it shuts down and only with WINDOWS booted.

and or/

win10 PE:

For example when I was booted up in Hiren's Windows 10 preloaded environment I did a stress test on the card.

finally a valid test, fails.  your boot HDD is not even need to do this test ,  so if that was true then the  HDD is not bad. nor its OS.

but there are more clues told. now.

When the EXTERNAL drives were connected it powered off.Failed, (what are those in detail??????)_

(so finally you proved its hardware and not the boot DD)

good. progress made and no need for CSV files and no Discord virus. great

When they weren't connected, it completed the test.

t. For example when I was booted up in Hiren's Windows 10 preloaded environment I did a stress test on the card.

When the drives were connected it powered off. (External drives were connected)

When they weren't it completed the test. (so the External unstated drives have problems)
can you tell use what EXT. drives you have and how connected, pretend we cant see your PC.

mine has 5 inside, and 2 outside,  1 external usb. and 1 EXT esata, what do you have.?

but when the card gets to hot,  did you run GPU-z  to see if the card overheated?

EXTERNAL HDD,

some are USB, if the HDD is 3.5inch IS IT, and as most do , they for sure overload most USB ports ever made.

many huge fast HDD have 2.5amp startup currents,  mine and  my hardware MATCHES it.

a good External HDD (have many all hand made by me with top grade fan cooled enclosures and a 3 amp 12vdc power packs, so that it never can overload any USB port made.  do you do this wrong.?

Some PCs, the USB  ports rated at 500Ma,  if you overload it , it only shuts down the port, (not pc)

other PCs have power monitors on the port for power overload, and the PC shuts down.

NO EXTERNAL  HDD stated by you, what make , want brand, what size, GB and is it 2.5" 3.5" or SSD drives.?

vast  types exist, mine are all power hogs (fast and over 3TB, Enterprise grade Seagate's) and use lots of current.  what do you  have?, I could never guess, external.(means so little )

but bravo YOU  , we have a hardware failure here. and seems not heat related. so far.

progress.

KOOL

did you first do tests proving its is software or hardware problems.

the tests are easy, why not do that first,  then we can work the true path. see?

if confused how to .. then ask.

do not put links to infected web sites .

why  not install a  SSD drive and load windows 10 fresh,  see it it runs for a week with NO APPS loaded by you.

that would cost you $20  to do that w10 is free for 30days, so why not?  why not learn to do simple tests like that.

or boot to Linux live media, in Try me mode, and run for a week and learn if your HW is bad.

POSTING:

that is correct , when you post it can fail, the servers here get overloaded time to time, .

or some times it tells you post failed, then shows double posts later.

that is same reasons. server overload.

and no posts to infected web sites, (endless there are now)

be patient it is free, to post here, so, be happy.