I get the exact same stutter on a GTX460 and 3.2GHz C2Q, even when both cpu load is well below 25% and GPU load is barely even registering in GPU-Z. It might be because of vsync combined with the framerate of the game being ~29.4 fps, which doesn't line very nicely up with 60hz (or 50, 75 or 120hz either for that matter) monitors. I don't think it is possible to avoid this without SE letting the game run at higher framerates.