I've sewarched the forum for 20+ minutes and not found a thread on my problem.
I have a 17-85 USM 4-5.6 IS that hunts for focus. It hunts about 3 times per second and never locks for longer than 1/3 second.
It does this in all light conditions (bright and dim).
All apertures large and small
All zoom lengths (wide-tele)
All camera modes (Single, AI)
Both cameras (T3i and 20D)
All selected focal points or individual
All camera modes (green box, Tv, Av, P, M, etc.)
Contacts appear to be clean and not damaged.
Does anyone have experience in fixing this or can someone point me to a related thread?