It could be your q-fan or it could be your overclocking. Try running at 3.8 and see if problem persists. I've experienced the same thing with my Core 2 Duo in the past. I had it running at 3.8GHz stable for a few weeks. One day, I returned home, powered on, BIOS told me something about PC crashing due to overclocking and FSB dropped to 233 (default).