marcm1, It may be a silly question, however what shutter speed are you shooting at. Keep in mind, with the 70-200, at 200, with no IS, the slowest you can handhold shoot is 1/200. If you are shooting indoors, it can be difficult balancing fast shutter, aperture, and iso. Also assuming you are shooting at high ISO and high shutter, what NR do you have your camera set at? In any case, do you use a supplemental flash unit to add more light or just shoot as is available light? It is easy for anyone to throw darts at the board and give advise without having a chance to be there to help you but either way, check those settings, shoot outdoors which may eliminate that low light situation JUST TO SEE if it is the lens or a camera setting. IF the lens still cant handle plenty of light and all AF settings are reset as needed, then it's the lens. If it works great outdoors or at least you can get a proper MA and good repeatable autofocus, then it's a light issue. I hope this helps