this problem also happend to me from time to time. what i usually do is -clean out all the hardware get all the dust out and make sure chip and heat sink are connected properly. also check all the cables are connected properly. - if still happen, I will use the software computer maintenance software to repair some errors in the harddrives. program such as winaso or any registry checking programs.
That was my next guess. The power supply might be at fault. Does the system automatically restart or does it just shut down?
Well, pulled the processor out, cleaned it and reinserted. Works fine for now. Anyway I'm gonna run it for hours continuously, let's see what happens...