I have a refurbished Canon 28-135 USM IS lens I bought from B&H a few months ago. It worked well at first with my Canon Digital Rebel XSi, but lately the autofocus seems "confused." It can't focus on most objects. It will focus in almost correctly then focus all the way out like i doesn't know what to do. Then if I keep on pressing the shutter halfway, it will try to focus again but fail. The problem occurs with IS on and off. I have to use manual focus to take a shot. I tried the lens on an older Rebel 2000 film SLR and it worked fine with no autofocus issues whatsoever. I tried another lens on my Rebel XSi and the autofocus on that functioned perfectly as well. So it's only a problem with the 28-135 lens on the XSi body. Does anyone know what the problem could be? The problem itself began during the lens' three month warranty, but I didn't have time to diagnose it until now because of my busy college schedule. Could it still be covered under warranty because of when it began?