USB cable (and cabling in general) may have much more effect than you think. They tend to capture the main and other sources of perturbations (the motors, your mobile phone..). If you have more than one connection (or other grounding of the camera via the fixation), you may also create ground loops. This may affect the camera (especially during reading the CCD which is very sensitive, and probably more so with low end cameras). Some USB cables may have less than adequate wires for the power/ground, increasing (or decreasing) the effect. Some contacts may be dirty.
If the banding is not always at the same place, you cannot remove it by darks.
Maybe make sure that there is no ground loop, check if using a cable with a EMF protection (as you may have for your camera) is helping, try to route cables differently, check if this happens also when the mount motors are not running.....
This are just random ideas, as if this is more and art than a science.
-- bitli
