Red Dots with horizontal purple lines

Which version, such as "SFF"  "Minitower".  Always include that with any initial problem post.

Assuming the monitor is not the problem, if yours has an added (separate) video card with the PC powered off reseat the video card (unplug then plug back in).  If you do not have an optional video card the problem is on the motherboard.  Considering the age of this model best option is to consider a new or at least a newer refurbished PC.  Many refurbished PC's (such as 4 digit model number Optiplex models) come with Windows 10 installed and prices as low as $200 US.

Thanks for the reply. This desktop doesn’t come with a dedicated video card and so as you suggested the motherboard have an issue. 

Integrated graphics is part of the CPU.  You might even try reseating the CPU and see what it does.

If it hasn't been done within the last 5 years, replace the size 2032 CMOS battery.  Duracell or Energizer is suggested as Sony only lasted me 2 years.  I get better deals tbrough Amazon vs. in store.  This isn't a cure all, but can often be where to start.

One user was getting a bunch of bizarre error codes   Once the battery was replaced, it gave us an error code that we could troubleshoot.

Thanks for the suggestion. I switch off the power, remove the cmos battery, press and hold the desktop power button to purge for 30 secs and inserted a new cmos battery. Sadly it didn’t solve the problem. 

Remove all of the RAM and power on.  Does it still give the same screen?  Then it isn't the RAM.  If you get a different screen, insert RAM chips one at a time until fault reoccurs.  I do have to mention - remember to power off and unplug anytime working on the inside of the PC.

If it's still the same, reseat or try a different CPU.

Still looks like a motherboard problem, to me.

Definitely quite possible, but at least we can do some troubleshooting that gets it down to the MB if that be the case.

The sad part, the only way to determine if its the motherboard is to replace it with another known good one.

The 980's are 2010 era machines.  Putting $$ into repairing a 10 year old PC is questionable.

MB should be cheap if NicGuy2006 chooses to go that route.

I’ve change the monitor, replace the ram, unseat the cmos battery and tried all the mentioned troubleshooting steps. The same result shown which is red dots on the monitor. Have done all these troubleshooting steps, the motherboard either need to be replaced or repaired.

True, it's down to the MB.  I don't know that it's repairable.  Chances are you can get an MB thru ebay much cheaper than a computer shop.  Be sure to get at least a 30-day return policy.

Re-seating RAM one stick at a time not working to see if there's a bad stick is news to me.

https://www.dell.com/support/article/en-us/sln284238/how-to-diagnose-and-resolve-common-memory-issues-on-a-dell-desktop-pc?lang=en

Dells have a diagnostic mode where you check 1 stick of ram at a time in the first bank.

You do not put in more than one at a time.

If you test all ram one at a time and it doesnt work in dual channel mode then there is an issue with rank and speed and vendors.

You can use 1 at a time but with pairs they have to match.

You cannot mix speeds and ranks and density etc in a single bank.

I'm sure you know diagnostics can't be ran if the monitor screen remains messed up; unless it magically comes together when entering BIOS upon startup.