What are you running this in? I did have some troubles running a 7XL with a MM in heavier vehicles. I couldn't gear small enough. When I went to an 8XL things got much better.

On the other hand, Finedesign has a reputation for selling crap and then not supporting it either. I can speak from experience.

Are you sure that you didn't use screws that were too long mounting the motor? Screws that are too long can mess up the windings in the motor.