start by opening the case and seeing if your pc has a lot of dust and that the fans are working...heat will cause the system to over heat and slow down. i would then see if you have some bad ram by running memtest. you can also run seatools or wd tools to check your hard drive. video cards when they start to fail will show weird colors on screen or no video.
i am getting about 50-30 FPS minecraft and 10-20 FPS battlefield 3
would this be a graphics card issue?
My system also Freezes quite often doing anything stays frozen for a minute of 2 then goes back as if nothing happened. if anyone knows the problem to this issue please tell me. i tried a brand new windows install with no result.