Computer acting strange

Alright, so this computer was brought to me, and I was asked to fix it. I started it up, and initially saw nothing wrong with it, so I preceded to messing around on the internet. Then, randomly, the screen turned black. The computer and monitor still had power, though. So I pulled out the power plug, waited a few minutes, plugged it back in, and went on it to see what went wrong, and after about 20 minutes of looking, I found nothing. Then it happened again. At first I figured it was a problem with the video card or driver, so I just installed a new video card and the appropriate driver. It happened again. I figured that it could have been memory as well. I then tried moving the two ram sticks around in the available slots and trying them one at a time, I even tried a random one that I had around, the problem still persisted. Just this morning, I reinstalled windows (and formatted the hard drive), and it's still happening. Does anyone know what's wrong or how to fix it?

 

I ran the disk cleanup and defrag system tools and also ran an anti-virus and anti-spyware program before doing anything with the hardware/formatting the hard drive.

 

The person who gave it to me just said that it was "broken" so I was asked to "fix it" in the general perspective.

 

I'm surprised no one has suggested either a possible thermal problem, power supply problem, or it could be that the motherboard is on its way out. By formatting and reloading Windows you have ruled out any software issues. By using 2 different video cards you have ruled out that as a cause. Memory is still a possibility, but not likely as 3 different sticks do the same thing. Hard drive and CD drive problems generally don't manifest in this way, so IMO those are out. What's left? Motherboard, CPU, power supply, and a possible thermal issue on any of those. Power supply issues also usually do not act this way, but it is possible if the voltage were fluctuating. Trying a known good power supply would answer that question. That would leave the motherboard (the most likely cause) or the CPU. CPUs fail somewhat rarely, but overheating can cause all sorts of craziness. If the BIOS has a shut down or warning setting for when the CPU hits a certain temperature, set it fairly low and see what happens....
Good luck, and welcome to Major Geeks 152217 :major

 

I'll try that. If it doesn't shutdown relatively quickly, I'll hook up a new power supply and try that out.

And yes, I have used multiple monitors.