Hello. I have a small problem with my TF2. When I start playing, it goes 60 FPS, just fine, but when I play for like 5 minutes the FPS slowly drops to like 40, which is kinda annoying and isn't smooth as 60. But when I alt+tab and then go back in the game, it's back to 60 FPS. Is there a way to keep those 60 FPS ? Thanks for your help.

My PC specs are:
Windows 7 x64 Premium
nVIDIA GeForce 630M-1GB
1TB HDD

12GB RAM
Intel Core i7