It's the sun
Mumblty years back we had a Compaq Desktop machine in one lab that would work for a couple of hours and then crash. But not every day. Bring it back to IT and it'd run all day. No magnets, no radiators near it. No damned good reason. Compaq came out and tested it. No fault found.
As spring turned to summer it got worse. One of the guys looked in every day and eventually it ran all day for no obvious reason until he worked out it was raining outside. We had a couple of rainy days and without telling us he'd worked out why, he'd tell us the machine would run today. First sunny day came along and he told us it would crash. Which it did.
Turns out the sun was hitting the black case, conducting just enough extra heat into it to warm up some microfracture on the system board and click. It would go. Same would have happened if it had been near a radiator or if the fan had been slightly blocked.
Compaq engineer came in, we put it next to a radiator, it went click and now failed tests so they happily swapped out the board.
Toughest diagnostic I ever came across.