Originally Posted by
Haleth
I've got a 6970 too (mobile version), they're pretty good cards.
You probably have vertical sync enabled which sync your fps to all or part of your screen's refresh rate (60 fps at best with a 60Mhz refresh rate, as soon as your card can't handle that it ducks to 30, then 15, etc). It also makes sure that only fully drawn frames are shown on your screen by allowing frames to be drawn in the background, so that you never see overlapping images or 'screen tearing'.
Tl;dr: As soon as your fps would hit, say, 58, it will fall back to 30 so it stays in sync. Lowering a few settings just slightly should keep it on 60 at all times. If you don't notice a great difference without vertical sync though, you can leave it disabled.