Hello, my xti is working perfectly until I put my 70-300mm lens on then it won't focus. It tries but the lens just keeps going in and out and it won't take a picture. It will if I manually focus it. The lens is a quantaray and I have used it many times in the past. Hasn't been dropped or damaged or anything. It still works fine with my 35-135 lens. Is there a certain setting I should check and adjust or does this seem to be a lens failure?