Sounds like my problem. This didn't start until after I had removed the rear wheel for first time. It is slightly easier to get the wheel off and on if you remove the motor, but it's not easy either way. I have re-tensioned the motor in the mount several times over the last few months. Every time I do this, the problem SEEMS goes away for a few days and then comes back. Hard to say for sure because it doesn't happen every day. I'll tension the chain and really tighten down those mount screws. Thanks for your help!
BTW - I'm starting to get the throttle issue you mentioned in another post. My fault because I jammed test leads into the connector to check the throttle pot. resistance and now the connector doesn't grip the pins as tightly. Looks like I have a couple of projects for this evening...