As Mcninjaguy said, it sounds like a heat problem.
If your up to it the challenge, you could confirm this theory by doing a little experiment. If you were to download, install, and run SpeedFan while your playing the game, we could see how hot things are getting. When it starts to lag, quickly quit your game and write down the temperatures of your CPU cores and GPU as displayed by SpeedFan.
Here is the link to the download:
http://www.almico.com/speedfan437.exe
Good luck, and please let us know how it goes!
