on my other computer, every time I play my game my fps is 60 or above and after 20 minutes of game play the fps lowers down to 10 and lower. Then I just can play the game.
If that is the problem there you can go for a card cooler to keep the temps down. You may want to check a few other temps as well with something like SpeedFan. Some will take a side cover when gaming and point a small fan in to see if there's another problem being seen. What are the specs for the other case?