simple fixes first, most issues are not the game, but infact the weather is getting hotter and odds are your fan on your GPU is getting slower with time.

If you have a ATI download this program you can manually speed up the fan. Odds are you will find its only running at liek 50%.

http://downloads.guru3d.com/download.php?det=725

You can look around thier if you have nvidia links their as well.