Low frames in batman arkham asylum

hi everybody,

I've been getting real shitty fps in BAS lately.
setup:
E8500 @3.8 ghz
GTX 275 OCed
4 gigs ddr2

first time I played it it gave me 70 fps,
second time: 11.

I tried to force it into 3d mode with rivatuner, that seemed to work, getting around 30 fps in 3D with physics.
but now, 3rd time I play without 3d it gives me 22 fps.

anybody know what could be the problem? It's rather annoying.