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"I have had this broom years and it's only had 6 new handles and 5 new heads. - Trigger, Only Fools and Horses."

I once got sent to a job, not without a little trepidation, because at least three other techs had been to this particular PC multiple times and from the records, every last part of the innards had each been replaced at least once. That left two options, so boss sent me out with a mains tester that would record spikes and drops etc. Rather than plug the device in and spend possibly quite some time doing nothing in the hope that "the event" would happen, ie the PC would reboot, I decided to gently tap the PC on each of its sides. It's a mini tower and the slightest knock on the side nearest the motherboard cause the now infamous "event". So I took the motherboard out, complete with it's mounting tray (remember them?), and looked down the gap and sure enough, the tray is bent and almost, but not quite, touching the solder points of the board. No doubt a temperature change would cause the metal to "pop" or maybe a kick from the user moving her feet around was causing the random reboots.

Remove board from tray, bend tray the other way, reassemble and all is well with the world, the universe is in balance and I never had to go back to that site again. Shame really, the user was very nice and I'd have liked to get to know her a bit better (I was still single back then!). I sometimes wonder if the other techs thought the same and that's why they never found the fault.