Why does my FPS decrease so bad?

Anyone can help me with this?
My FPS on Countestrike decreases quite badly, from 90 to about 35 at times. I dont think its the problem with my hardware:

I have:

-AMD 1.4 Ghz
-512 RAM
-Windows XP
-Radeon 9000 128 DDR

Just wondering if its my Open GL. But somehow i dont think it is, as I tried using Direct 3D, and it still decreases. Furthermore, it even decreases at the same rate when i play a LAN game. Strange. Any help?