FPS suddenly dropping during gameplay

1Carss1

Well, the tittle basically says it.
So, sometimes when I'm playing, my FPS will drop from 50+ to around 5-13, and I don't know why. Sometimes I can play for hours, and then it'll happen, other times it happens right away. It's really been making me mad, and I was wondering if there was a solution.

Thanks,
Carss