Not sure how to describe this, but the lens will jump when I turn the camera on and at random times while using it. I can see it through the camera and feel it in the lens. Its been happening for awhile now. My other two lenses do not do this. And this lens will do this on another body. I took it back to the camera shop once and they didn't see it happen. It is happening more now. Anyone heard of this and how I can get them to see it or describe it to them?