This past Saturday I was coding and my computer shut off on its own. It proceeded to shut down a number of times throughout the morning before I finally broke out my vacuum. I took out my nVidia GTX 260 video card to clean it and found a nice healthy layer of dust. I gave everything a decent cleaning and put everything back together. When I powered my PC up the video card was running 10 degrees (Celsius) cooler! I also found that my highest temperature from running a game (Dead Space in this case) was 15 degrees cooler. (around 55)
I've often heard and recommended "reboot your pc" as the obvious answer to most problems. Now I will add "vacuum or blow out the dust in your pc" to my list of simple recommendations when strange things come up.