On this lens I have never seen this before – but the symptom was absolutely the same like on the 17-85 lenses, so I decided to “refurbish” the USM motor.
At first, You have to take it out from the lens, and to disassemble from it the lower part, which covers the optical encoder.
After this I always make some signs on one end, to know on which position to mount it back. After taking it out from the holder, You can see that the transparent acrylic glass or polycarbonate is full of oil – which make o optical short circuit in the encoder. I usually wash it out with a kind of white spirit (or brake-cleaner from the car-industry), after which the encoder goes back to it’s place.
Here You can see the oil.
After assembling the focus worked perfect!