Since this is an individual circuit it should be pretty easy to diagnose. At the panel (with the breaker off) check for continuity between the ground and neutral when disconnected from the bus. If it is anything but infinity that is probably the trouble (ground neutral short). Work your way back to the motor until you find it. Next step would be to temporarily hook it up to a regular breaker and hang your clamp on across the ground. It should stay zero. If you have something, I would suspect an intermittent grounded motor winding (real ground fault).
