My first thought was that your CPU was overheating, too. If not a heat issue, maybe the PSU is dieing on you? If you can just get to the bios screen, you should be able to check temps, and possibly voltages from there.
Supposing it is the power supply what signs should I be looking for in the bios voltages and there any way to be sure without completely changing out the psu? I could put mine in there but removing mine from my pc and putting it in his would involve a fair amount of effort and I don't have much free time