Have you lifted up the heatsink recently? Is it hotter in your room? And there is always the chance that your temperature sensor has gone, and is now reading an incorrect temperature, and causing your PC to shut down to protect its self.
If you don't mind spending a bit of money, you could get a new heatsink. Or for less, re apply the thermal compound. You will need to clean the old stuff off with isopropyll/anything 90%+ alcohol.
Cisco CCNA, Comptia A+, 1/3 through CCNP