FPS drops

[Bounty]

I set up my FPS to 40 (it's showing 45 fps anyway) but sometimes It's dropping to 20 and I need to restart my game to get 40+ fps back. Can I do something about it?

[Bounty]

Ok I found out how to half-fix it.

Just disable full screen mode and when fps will drop below 40, minimize game for a moment and go back. There should be 40+ fps again.

But if you have any ideas how to completely fix it, let me know.

[Bounty]

I just wanted to mention...that this problem disappear. There's no fps drops anymore. I didn't change or install anything...except updating my graphic driver. Maybe that's why.