RiverPhoenix

I am able to run any pc game on ultra settings with no trouble at all but this is my problem: I get a contant rate of 60 frame rates but every 1 minute or 2 the frame rates drop to about 17-18 and the game lag really bad but there is no difference in visuals then after about 15 seconds the frame rates shoot back up to 60 again and the game is back to normal? Why is this happening ive tried everything possible to fix this for nearly 6 months now its sucks! Games i play are Far cry 3, dishonored, War Z, Black light retribution, Diablo 3, Assassins creed 3 etc.. specs are: AMD FX 8120 8-core 3.1GHz cpu ASRock 880GM-LE FX motherboard 8 gigs of hyper kingston ram 1TB seagate HDD GTX 660 TI Cooler master 700 power supply And i have 4 fans in the system too thanks