Originally posted by RobA_Oz I had a similar problem with mine, some years ago, after it had a little fall (about 100mm) onto sand, while attached to my k20D. I took it to a repair service, who told me that the cause was the little steel ball in the helix jumping out of the track, and that I shouldn't attempt to repair it, as they had manipulated it back into the track, and that the damage to the track would self-repair with time. They were right. I'd completely forgotten about that until this thread, which demonstrates that the use of plastic for some things isn't always detrimental. Gentle movement of the zoom ring, back and forth, was all they did, until the ball popped back into the track. I hope your issue is as easily solved.
My lens did go in for service 2.5 years ago for a similar situation. It was stiff back then too. But it freed up after a while. It only started getting noticeably stiff recently.
I guess it'll go in for service with one of my K5's monday.