FPS Problem

I think a lot of people got same problem with me, because i asked 10 of my clan-mates and they all answer the same: "FPS are sh^^,and all the pings are hell"
....Okay, my normal FPS is 40-50, and after 5-10 minutes, my FPS goes down like 10-15, and how can i play with that FPS ? it keep spiking up and down like heck ! whats wrong with CF ?