I've never known a CPU to partially work, or haven't experienced it yet! I was in a similar situ' with a Socket 939 mobo and CPU combo and like you running an OC. But the CPU completely gave out. I didn't suspect the mobo, A8N SLI Premium, and after swapping the CPU with a back-up it worked fine till the day I sold the lot on eBay.
I'd try and get hold of a cheap AMD Athlon 64 1.8Ghz CPU to test the mobo if you can.
