I am using a used Canon 400mm 2.8 L IS (version I ) for last 3 years but have recently developed a problem. The AF start hunting and it keeps on going forward and backwards at one point but don't give a focus confirmation beef. It won't AF at all and I have to manually focus to take a photograph. After sometime AF would start working perfectly fine. It kept on happening for quite some weeks and now it is consistently giving that problem. Anyone else has gone through similar problem or any idea what the actual problem is so that I can have a fair idea before I send it to Canon for repair.