This is probably the result of the H.264 encoder. The 350 switched from VC-1 to H.264 as it's more efficient at compressing video streams; but it requires more processing power to decode. So, you may be getting dropped frames from the H.264 encoder, which wouldn't be all too surprising...or your PC could be dropping a frame or two of very fast motion as it's easier to skip it than try to decode it.
I do believe the player was decoding every frame...and extremely fast motion was causing the video stream to hang up and brought up the audio lag we dealt with. It now drops and skips frames to keep the AV sync'd up.