Something intermittent but temperature sensitive, I'd say.
Having been there a fair few times, these are the final causes as found:
- Dead/seriously ill power supply (six or seven of those, on four computers).
- Duff IDE cable from hard drive to motherboard (two of those).
- Dead or dying hard drive (two of those).
- Duff USB device (cheapo webcam).
The power supply faults included a loose molex in one case, a dead fan in another, and a well-exploded power monitor chip in another - which may have been caused by something upstream of it.
My "first IA" now is to swap out the power supply (I keep a spare, big one in the cupboard). Even if it's too big to fit in the case of the particular PC, it eliminates one option. So far, it's always proved that the resident power supply was defunct.
A 1kW power supply can fail just as easily as a 400 watt one, so the fact that it's rated at far more than the PC needs is no guarantee.