Original post by: Nick ,
If the RAM slot you used was empty before, it could well have oxidised contacts and be full of dust etc. I would reseat all RAM twice before reassembling. If this doesn't fix it, I would suspect faulty RAM. If you suspect overheating, you'll need something to excercise the CPU. I've had one of these fail when applying OS updates purely because the fault only came out when it was working hard.