There is nothing recorded in the hardware log?  Have you checked OMSA from the OS?  Without anything in the hardware log, I would suspect a processor or motherboard issue.
We've got a PowerEdge 2950 running Windows 2008R2 x64 that's suddenly dying about 4  minutes after power on, even if we stick in BIOS. Any suggestions? We've tried swapping out power supplies to no avail.
PE2950 dies a few minutes after power-up