I was adjusting my gears today in my new stand (thanks to wiggle and those on here), and it was definately easier. However I noticed that when I stopped turning the pedals but that the wheel was still spinning, that the cassette sort of wobbles as if it doesn't sit correctly. Not by very much. I don't think it feels loose. The wheels are new (less than a month old), but the cassette is old. The wheels were replaced by the bike shop. Any ideas?