I've seen this problem where too much thermal grease has been applied. There's the oblong shape of the chip, which is where the grease should be. Around the oblong, there's a few metal contacts which must not be covered in the grease. I've found this more and more lately when diagnosing weird faults. I think it's because users are becoming more conscious of heat affecting their PC.
The only other suggestion I have is faulty memory. Can you borrow memory to try as a substitute?