All games run at low FPS after computer has been on for a while.

Is there any way to prevent this from happening?
I can easily restart my computer, but this becomes tedious when I need to restart my computer every time I want to play a game.

Here are my specs:
Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Ultimate, Service Pack 1, 64 bit
Processor: AMD Phenom(tm) II X4 970 Processor, AMD64 Family 16 Model 4 Stepping 3
Processor Count: 4
RAM: 12286 Mb
Graphics Card: NVIDIA GeForce GTX 560 Ti, 1023 Mb
Hard Drives: C: Total - 476937 MB, Free - 4080 MB; D: Total - 1907726 MB, Free - 1280440 MB; E: Total - 953867 MB, Free - 159545 MB; G: Total - 2861458 MB, Free - 78144 MB;
Motherboard: Gigabyte Technology Co., Ltd., GA-890XA-UD3
Antivirus: Microsoft Security Essentials, Updated and Enabled