I would suspect BIOS built in heat threshold shutdown.
You mentioned your HeatSink was very heavy and that this problem only began after your move.
I would unmount the HSF, clean the bottom and the top of the chip, apply fresh thermal paste, reseat the HSF and go from there. Including reviewing BIOS settings for Thermal shutdown behavior. Even if there are no configurable settings you can see, it doesn't mean there aren't some built into the system.
Hope this helps.