IMac :: Slow Game Speed And Dead In The End?

I just installed windows 7 via Boot Camp, and then installed the game Left 4 Dead 2 on it and started to play the game, and everything was going well until about 20 minutes of playing it.

The game soon started/continued to slow down, or have that lag effect, however, let me note, I'm playing this offline "single player mode" just to make sure it had nothing to do with my internet. Also let me note, nothing else is running on the computer at the time, no updates, nothing.

So then I touched the top of the monitor, and it was extremely extremely hot, and of course since I'm playing a game the GPU will produce more heat, but the only thing I could think of is, the GPU is becoming too hot and effecting my game play. I know most say Apple is very smart and heat should not have an issue, but what else would cause this as everything else on my iMac works fine, and when I close out the game, everything works alright. Videos play fine etc..

I haven't heard anyone else have this problem so I wanted to ask you guys who are experts here to tell me what you think it is?