New
#11
Ubuntu is just another clean OS; the same effect as doing a clean install of Windows.
Since the stress test seems to be running okay so far, it throws doubts on whether the CPU is actually defective or not. It seems that a problem so serious would show up quite quickly on Prime95, if indeed the cores are bad.
If this was a heat problem, Prime95 should trigger a crash within minutes (if not less).
It's sounding more like a BIOS problem than anything else; is it possible you could get a new motherboard with the same socket? Perhaps visit your local PC shop; have a chat with them and explain the situation. Perhaps they will let you buy a motherboard, and allow you to return it if it doesn't solve the problem.
Again, we could be way off though. Is it possible you could just try the old CPU, to make sure it is indeed the new CPU causing the problem? Other possibilities include bumping the RAM, hard drive, zapping the motherboard with ESD, etc.