Random and sparatic FPS drops

During games my FPS will randomly flux ate between 350, and 50. This happens very quickly and often causes the screen to freeze for a second or two during the changes. It's becoming increasingly difficult to play and it only stops being a major problem about 20 minutes into game.

Sometimes it will persist ALL GAME. Sometimes it will only be for the first few minutes.

This is definitely not a hardware problem because usually my game runs at 350+ FPS but now it just runs at whatever it feels like.

I've checked everything, this is deff a problem with the game.

So how do I fix this?