Batman AC Physx Issue

I'm having a rather large problem with the physx in batman arkham city. Any scene that is heavy on physx, my framerate drops from a fairly stable 60fps to as low as the low 20's. This happens with it set on normal or high with dx9. I'm only using a single 560ti so I expected some slowdown but I don't think it should be dropping by that much. I decided to monitor my gpu with afterburner and during physx scenes the gpu usage can go as low as 50-60%. Any thoughts on how to improve performance with physx?
Win 7 64
i7-870
4 gigs ddr3
gtx 560ti (latest beta drivers)
onboard sound
700w 80+ psu.

i had to turn physx off in the end up to make the game play properly

even with

i7-2600k at 5ghz
16gbs ram
overclocked gtx570

If I run the game in dx11 it's unplayable due to very low framerates so I stick with dx9. I've seen posts in other forums with people describing similar issues with far better systems then mine so i'm not sure lack of RAM is the culprit.

even on dx9 i had problems

the game played fine but my saved game would corrupt sometimes and nothing would fix it--and would have to start again

turned out to be physx was the problem-turned it off totally and went through the whole game fine