Low FPS in game With ATI 6870

Jessy*_*

Hello guys I have a FPS problem here is my PC pecs maybe some one can tell me what to do or what the problem is... All the drivers are up to date...

Intel Quad core 3.5ghz
8gb of ram
ati 6870 video card
Widows 8.1

My fps is low and i get lag...

FPS range: 40fps - 100fps

Please HELP!!!

Thanks...