If its any help, i had a Canon 400 which was a frigging nightmare, wouldnt focus on anything, had it calibrated, serviced, calibrated to a brand new MKIII and an older MKIII, was *****, sent to Lehmanns, sent to Elstree, took it back to Lehmanns, finally decided to use the Calumet drop off point in Manchester and hey presto, a guy in there used to work at Elstree and asked about mjy problems, he asked if he could have a look at the lens as he thought he knew the problem, he came back 10 mins later and told me the IS motor had bust and even though i never ever used IS and it was always turned off it still slightly whirred away inside the lens and while doing so threw off the AF syytem, he sent the lens off with a note and hey presto 4 days later a brand new IS unit and a lens that focussed bang on at every aperture and every focal length.
Mind you i then ditched Canon for the first time and went to the dark side simply because they were incompetent.