There are MANY FPS drops and note that the% use of FPS varies greatly.
Some procedures have already been taken (refresh driver, make sure it is not warming up).
My Setup :
Athlon X4 860k @ 4.0 GHz with Water Cooler Seidon 120V
8gb DDR3 1333
Motherboard MSI A88XI AC
Bitfenix Prodigy mini-itx