Before I commit $$$$ to professional advice, perhaps someone might have a quick answer to this problem. Have an EFS 17-85mm IS lens which has a malfunctioning AF system, the symptoms are identical to using the AF on a lens which is in perfect order when trying to focus on a plain white wall or when it is too dark in a room for the lens to get a correct read on the focus point.
I bought the lens at a decent discount knowing that the problem existed, thought a change of camera body might fix it but no dice there. Tried cleaning the contacts and same result, no joy. Then tried another 17-85 on the same camera body and perfect focus!
The focus system continually works its but off trying to get a correct read until you zoom right out and then "half-press" the shutter button, AF then works fine for a couple of shots and then reverts itself to focus in and out, in out.
Probably a busted chip in the lens, me thinks.