Just wondering if anyone else has noticed that when manually focussing their 70-200mm that it sometimes "slips"? ie you turn the focus ring and nothing happens? It tends to only happen if I turn the ring quickly, which is rare, but it's still annoying. Is it just my lens or is this part of the design? Everything else works great, just when I focus by man rather than by machine. I've also noticed that the AF seems slow, tho I don't really know for sure as my only reference point is my 17-40.