... A temperature-dependent bug like this just reeks of a hardware
fault: either dry or failing solder joints, or a component at the
ragged edge of failure.

I'd wait for the temperature to rise, and then spray specific parts of
the board with "freeze" (spraycans containing compressed refrigerant).
With luck, you can narrow the problem down to a single area or a
single chip.  I'd suspect the cache, or anything from the CPU to the
memory-bus parity glue.


Caution: This may be tricky if you cant access the underside of the
motherboard. Also, the thermal stress may make the problem worse.