There was aline of laptops a couple of years ago that had bad capacitors on the mobo adjacent to the cpu ... Sony I think.
I just had to replace a desktop motherboard (EVGA) that had a similar issue. Speedfan reported the cpu itself was within spec but the board sensors adjacent would overheat as soon as cpu usage hit about 40%. So the machine could surf along for a long time and then crash after a more intensive app was opened.
Either way, speedfan should give you clues.