Crysis low fps on high settings [Radeon HD 6850]

I need help. When i play crysis 1 i have lag on high settings. And my latest discovery is that while on the ship when i fight a giant alien spider as i like to call it, the game frequently freezes and after a couple of seconds it continues working and after a while the whole PC freezes and i have to restart due to it not responding to anything i press on the keyboard. I see GPU usage is 98% and CPU usage never goes near 80%.
PC spec:
R 6850
athlon II X3 445
4gb ram
antec HCG 620W
160gb HDD
biostar n68s3+