My Nikon 20~35 f2.8 zoom has developed a problem. Just to familiarise you with this lens; it employs a manual/autofocus button on the lens. Once AF has been selected on the camera body, AF can be engaged/disengaged on the lens as necessary. Whilst using AF, the focus ring rotates freely. Trouble is, after using AF and returning the lens to manual , the focus ring doesn't 'pick-up' the focus gear inside and continues to rotate freely. After turning the focus ring for some time - first one way and then the other - it will eventually re-engage, but it seems to be getting worse and taking longer each time I use it. Curiously, it seems to be more troublesome when it's cold. Any thoughts? Many thanks, B.