Definately sounds like a mobo problem. Dying or faulty processors usually manifest themselves in BSoDs within the Windows environment or failure to install Windows (it should be noted that a processor will many times load Windos 98 just fine, but if faulty, will fail when trying to run Windows XP). I just fixed my long-time headache of a computer problem - guess what, it was the motherboard. In my case, it was just causing general instability, BSoD, and major headaches. Definately try swapping the motherboard, fail-safe defaults are another thing you can try within the BIOS, but if it doesn't run your parts to spec, get it replaced.
Regards,
Jarod