Constant speed for an unbalanced load

I have a machine that uses a Dart 253g to drive a dc motor. The arm if an offset arm that rotates.. We have tried to balance the arm the best we could but the load is still off. What happens is as the heavier part of the arm is on the upswing of the motion, the drive cranks up the torque and pushes it up and over the peak of the rotation. The problem is, on the way back down, the load takes off and "freewheels' around until the upswing starts again. Would a constant speed drive help eliminate this? This machine has no feedback on the arm 

Thanks in advance 

Ken