Not sure if you have solved your problem, but this info might help you smooth out your process. I have encountered erratic feedback signals that have caused undesireable fluctuations or contributes to oscillations. As mentioned in the other replies it would be best to try to filter the feedback signal, but if this is not available or proves in-capable why not filter the 4-20mA signal going to the VFD. I have used the the acceleration and deceleration times in a VFD to sort of prolong or damp out erratic speed reference signals. Increasing the accel and decel times in the VFD will add more time constant to the system but may give a more average weight. (Most VFD's have the ability to filter the 4-20mA speed signal as well) Hope this helps.