Need help with my fps

Hi,
1 month ago I bought a new computer, it ran this game at ~170 fps. Since a while my game starts lagging in big fights? :S also when I'm shooting sometimes. I have no idea what's wrong but my proccesor is only in use for like 50% and gpu too when I play the game.

Specs:

4gb Ram ddr3
Core i5 proccesor (4x2.6 ghz)
Ati 5850 graphics card

Like you can see my specs are 10x+ better than recommended specs. And on my old computer which was allot slower I didn't experience any lag either? Gamebooster doesn't work

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