Can you try resetting your CMOS? Pull out the little battery for a minute, stick it back in, reset the date, time, and everything, and see if it works.
You'd be surprised how often that works. I doubt it, but it's worth a try... doesn't sound like it's your CPU, really. Probably the motherboard... mine went out in pretty much the same way. It just starts going bad, and continually gets worse until it refuses to do anything at all.