I noticed your NVidia driver is over two years old, so try updating that. It does sound like an overheating issue, and 2 years is about that mark when so much dust will build up around your fans and heat sinks that it causes problems. I noticed you mentioned something about dust in your power supply, but that shouldn't be causing the issues you're having (but stranger things have happened with computers, and its good to clean it up so dust doesn't travel to more critical components).
Check inside your computer's case, clean the the fans and heat sinks over the CPU and graphics card processor. Take a can of dust-off (compressed air in a can) or a gentle dry rag and remove all the dust bunnies, if your not too sensitive, give them a good blow too (note: dust will likely fly back in your face).
If that doesn't work, then it could be as Skuzzy mentioned, where there are too many little processes running in the background while it's running the game (causing the computer to work too hard and overheat via being overworked). If this is the case, then you can get some good freeware to help you clean up and bring under control that problem.
http://www.alacritypc.com is a good one for managing the processes on your computer (not cleaning up or repair, which is a good thing to look into doing first), it can be a little complex but is fairly explanatory about each item you choose to enable/disable while running the game. The program essentialy stops anything running on your computer that you don't need/want when you opt to play the game. When you quit the game, it will then restart the processes it stopped or only the ones you want to run while your game isn't running.
