I had the same issue with a MMORPG. It started out of nowhere and was pretty random. After some investigation I found that the fan shaft on my video card had gotten gummed up and the fan was only turning a fraction of its original speed. GPU was getting hot under heavy load and shutting the computer down.
You might look at all your fans, heat sinks and surfaces to make sure there is absolutely no dust, pet hair, debris build up that could be causing some heating issues.