Here are some hints. Set current limit to nameplate rating. Check for proper field voltage. Check proper polarity for the series field winding. Adjust accel and/or decel. Adjust IR Comp to lowest setting, increase until unstable, then reduce setting slightly. Check brushes and commutator for excess wear. Check for mechanical drag in the load and the motor. Check bearings for wear in the motor and load. Many other things could be the problem, these are the first things to check.