Same problem, ATI 6850, Win7 64bit, dual core machine with 4GB ram. When it runs it looks great and is smooth - but it only runs for about 3-4 seconds at a time, then stutters for I'd say more like 1-2 seconds. Then resumes and repeats.
When I say "stutter", I mean a complete hard lock to the point of even cutting out audio. I have the same issue in ArmA2: OA, if that's any help. I used to be able to play OA just fine hmmm maybe 4, 5 months ago? I was on a 4850 though, so I'm not sure if that's even relevant.