Just thinking out loud...
How many amps are you showing in each direction with a empty car? With capacity? With a balanced load? You should see a balanced load between 40 and 50 % capacity. Remember when checking for a balanced load that your amps and your car speed should be the same in both directions.
Also have you meggered out the motor? Have you opened up the motor and checked the rotor. Broken bars will cause problems.
