Ok, I successfully changed my chain, and I'm relatively certain that the chain is the correct length. I put on a new cassette at the same time, as well as new deraileur pulleys (one of the old ones was broken). In examining my chain I found that if I spin the chain, either direction, the rear deraileur bobs back and forth. I can't think this is a good thing. So is there something amiss? Oh the motion is regular, i.e. at at constant frequency about 2-3 times a second. And related to how fast I spin the chain, faster chain speed -> faster motion. Doesn't do it freewheeling, don't know if that helps. Any ideas?