HD 7950 OverClock Help

Hi, Not sure if i'm in the right place here, i never used this. I have a problem Over clocking my GPU.
Im running a i5 3570K Ivy Bridge
ASrock z77 Extreme 4
8 gbs G.Skill Sniper RAM
Shaphire HD 7950 3gbs
I have tried Over clocking it to get a little more fps in Hit man: Absolution on Ultra, but When i use Overdrive or after burner, no matter how much i increase the cards clocks, it drops from 61 fps on 2X MSAA on FurMark, to about 30 Fps. I'm not sure why it cuts the preformance in half, any ideas?