Did you check all the fans (especially the one on the video card)?
I don't know what is wrong with my computer, I think it could be the video card. At first every once in a while after playing a game for about an hour my screen would go out and I would get a rapid chopping noise from my speakers. It would happen every couple of days, but it now happens after an hour. Recently it does the same thing for a short while, but after about 10 seconds it will just be a black screen and I can hear the audio just fine. I don't seem to have input control though because I was on ventrillo and my friends weren't able to hear me. At first I thought it was my video card (GIGABYTE GV-R785OC-2GD Radeon HD 7850 I am using driver packaging version 9.002-120928m-149042C-ATI and 2d driver version 8.01.01.1266.), but I am thinking it might be more serious now.