My first guess would also have been "mains fluctuations" (brown-outs, sags, interruptions ... whatever). And then maybe the power supply voltages.
How good is the mains supply where you are (where the machine is)?
But what about "heat related"?

When was the machine last opened up and cleaned (blown) out?
I'm also wondering if something extra is being (has been) plugged into the machine; a remote monitor, a USB memory stick, or what-have-you.
Anyway, no doubt you have got it sorted out by now ... so what was the problem? Hopefully it wasn't the catch-all "software corruption" ("re-install software"
etc.).
