What is the ambient temperature?
I've heard of several of the following:
- Cracked MOBO overheating and shorting out - but only when hot
- CPU overheating
- Clogged/dirty heatsink fins
- Overloaded PSU
- Dirty PSU
- Overheating Mem
On the non-temp side:
- Flat BIOS Battery
- Corrupted BIOS
- MBR Virus