There could be numerous reasons for this but it might just be that your PC is overheating due to an internal fan, possibly the one on your processor has failed.
The easiest way to check this would be to take the cover off you PC and start it up. There should only be two fans, one on the processor heat sink which will be on the motherboard (big square circuit board) and one in the power supply (square metal box in the corner with lots of wires coming out). They will spin all the time the PC is switched on.
It might even be the power supply itself intermittently cutting out. I've seen this happen and it needed replacing.
If the machines power just cuts out it is more likely to be a hardware fault rather than software in my experience.
Jeff Hooper, January 2011
