Do you feel the wheel 'click' when you move it? Could you be seeing the same issue that crept on some of our 40Ds a few years back, where the rubber isn't gripping the plastic disk? If so, you can fix that with a couple of tooth picks and a tiny, tiny dab of super glue.
The wheel is taking quite some abuse as I have the camera dangling on a blackrapid strap on my back. 

