Quite frankly, the 6950 is a middle of the road performer. A good card, but it will get stressed with all the new options.
That said, every computer is different. What bottlenecks one computer may not bottleneck the same hardware for another user. It really depends on the software installed and the configuration of said software.
I am trying to get to a point, at home, where I can do a full cross matrix of all the game video options to show how they could impact a system. I just moved so I am still getting everything together.
Other than low frame rate I have a problem with the graphic detail not staying set where I put it. The detail water is the only high detail I run all else is off. Yet after 5 minutes of play my FR drops 55 to 5 and the graphic detail will change. I reloaded this game three times and down loaded Game Boost to run it with and I still can't keep enough frame rate to play.
I'd need a DXDIAG output for this one. Off the top of my head it sounds like something else is grabbing the CPU, or the video card is really a low end card, or there is a video driver issue (not well matched to the card). Heat could also cause this. The power supply could cause it.