Display whack! don't think it's vid card

This is my first post, thanks in advance for reading this somewhat long description, but I figure too much info is better than not enough.

Recently, my computer display started to randomly display a full screen color, a random color each time, that locked up the computer and required a hard reset. I noticed the longer the computer had been at rest, the longer it would take for this problem to show up. ie, If i just had reset the computer, it could take as little as 3 min. First, updated driver...nope, next downloaded speed fan and gpu z to watch temps...mid 40's idle and i've never really been doing anything graphic intensive, at most just watching a youtube video. Just the other day, after rebooting several times (due to grey/green/pink screens) the computer refused to post, and instead i got one long, followed by 3 short beeps from my Asus p5n-d motherboard, indicating a unseated or faulty graphics card. So before dropping another 200 on a gtx 260 I borrowed from a friend an 8800gt and tried again.

It had been off for over a day when I booted it with the 8800 and it booted fine, loaded the drivers, then after about an hour (the avg amount of time it was taking for my 260 to give me colored screens before from a cold start) my active window started getting pixelated, and if i clicked on something else, it would become pixelated too...it became more and more grainy until i couldn't recognize anything on the screen, so i shut it down and tried to boot it back up, to only have it do the same thing w/in a few minutes. I had noticed that the fan speed had been varying the last couple minutes before the problems...and not according to temp/load, it was just spooling up and down, quickly. So i began to think about maybe a power supply. So i pulled it out and tested all the pins, and the voltage was within range.

2 questions: Is it possible that the power supply would start having voltage issues after running for a while and getting warm, and still be able to put out correct voltages during idle?
Does anyone have any other troubleshooting reccommendations before i just start throwing money at it to figure it out? I hate that technique, if you can even call it a technique.

oh, i forgot to mention that i tried a different screen. Sorry again for the long post.

 

This sounds like a faulty motherboard to me. You have proven that it is not the video cards, and it is very unlikely to be the power supply.