PC works then crashes

the pc had a blue light, I think you mean monitor screen shows blue video. If yes it is called BSOD. Typical of system instability and crash. First thing to check is memory. Remove all ram sticks then put only one ram back, test for system stability for 1 hr. If still crashes in 20-30 min, repeat test for the other 3 ram sticks until you find one stick that does not cause crash. Remember each time only one ram installed.

if none of ram work, try to get a replacement ram of similar speed to test again. If still not good, then problem is something else. Are you using a video card?

Have you tested each ram stick individually or tested new ram replacement?

adding a note that the pc crashes after 30 minutes exactly consistently.

OP means blue power light.

The first thing I'd do with the on & off problem is replace the size 2032 CMOS battery.  The 990 goes back to 2011.  Has CMOS battery been replaced within the last 5 yrs.?  It's a cheap fix even if by chance it doesn't work.  We recently had a 990 with constant on & off problem and the battery took care of it.  Noting your scenario is different.  Battery warning doesn't seem to show until it's nearly dead or dead.  Also, please see below.

According to Dell Diagnostic Indicators :

the biggest problem at the moment is the shutting down after 30 mins,

by shutting down i mean its asif i turned the power off at the wall, and by 30 minutes i mean after exactly 30 minutes consistently.

this led me to believe its either a software problem, malware or my motherboard, ive conpletely checked my pc for malware and its clean, i also coundlnt find any programs which would cause a timed pc shutdown,

and the motherboard is all that is left, if the time was around 30 minutes and could be altered depending on what i used the pc before then i would have thought of ram or battery but its exactly 30 minutes.

Then that must be a power management issue of pc. Look inside power management in bios as well as Windows.  Often you can set system to sleep, hibernate or do other rest like activity at regular interval.  If you see anything like 30 min, change it to 1 hour or longer and see if that helps. If nothing changes, reset CMOs jumper to clear settings if root cause is in bios, or do clean install of Windows if issue hidden in OS power mgmt.

Adding to what redxps630 said, if not already, make sure everything is set on "never" for pc sleep, hibernate, and shutdown.

i have checked windows settings & bios and there is nothing causing the shutdown, i tried replacing the psu since its what i replaced before and the new one could potentially be faulty, i also replaced the cmos battery but the only other 2032 battery i had was in a old pc we arnt using, it ended up still having problems starting (powering on & off quickly when power button is pressed aswell as going to a 3 on the led error code) but when it started up it stayed on indefinately till i turned it off at the end of the day (sevral hours) then when i turned it on the next day it ran for 30 mins then conpletely lost power again, i tried again and the same thing happened, then new cmos 2032 batteries arrived and i replaced them with the old one i had put in my pc and it still wont run for more than 30 minutes

Go into BIOS and leave the PC running and time it.  If it crashes, this rules out a software problem and we're back to hardware.

Remember, error code suggests CPU.  It may need reseating or you may need a new CPU.  But lets try my suggestion in pgh. above first.

Good idea. But first, in the bios, turn off things like "c states". And other settings like intel speedstep, turbo, etc. Anything that fiddles with the processor clock & voltage.

On my 790, it will loose conscieousness when running something overnight like a memory test (without windows booted) Unless I change bios settings. I think it was probably the C states causing this, but never investigated further.

Just a thought, since 990 and 790 are similar.

it only stays on for 30 mins in bios, the thing that is really confusing me is that i somehow fixed the 30 min powerout when i switched the cmos battery but then after that day its gone back to 30 min power down even with different cmos batteries