One other thing to check, lately I have found some of the dogbone ears fit tightly in the drive couplings. If the motor is not aligned with the gearbox it will bind. You might be able to check if thats the case here if you loosen the set screws on the flywheel and see if can move back and forth on the motor shaft. If not that could be part of the problem. The fix is to unbolt the motor mount from the frame and pull the motor back far enough to free the dogbone. Then just lightly file the ears down so they slide easily in the coupling. That and aligning the motor should rule out motor binding as the problem.
Pete
