strip unit down.
full clean and re-assembly
make sure no dust bunnies are left
clean and reseat memory
check all units are fixed securely [HDD - floppy - MoBo etc] all items that are loose are a threat to system integrity.

run software checking tools over the HDD
run software checking tools over the system complete [CheckIT - PC Check etc]
run the [your] mark 1 nose over the PSU - clue - if it smells 'burnt' replace.

security routine ?
uptime - unbroken, prior to this problem ?
backups ??????? got none - GET SOME

ps

what DO your logs show ??

[edit]
just seen your remarks re:- heat and stress tests.
still leaves the physical pull apart