FPS

So i downloaded the first FEAR game (one of my favorites) and when i start playing im at like 120 fps then like 10 minutes in it drops to under 10. What is this happening? Im not a big PC guy. I just want to play without messing with settings every 5 mins and every 20 mins when I start a new game. Any ideas to make any game old or new just run without all the hassle?

So i downloaded the first FEAR game (one of my favorites) and when i start playing im at like 120 fps then like 10 minutes in it drops to under 10. What is this happening? Im not a big PC guy. I just want to play without messing with settings every 5 mins and every 20 mins when I start a new game. Any ideas to make any game old or new just run without all the hassle?