Servo Motor Constantly Faulting- From Feedback?

I have a machine with a back-and-forth axis that consists of a servomotor directly coupled to a ballscrew, approximately 2kw. This is driven by an older Allen Bradley 1394 system(SERCOS) and controlled with a Controllogix.

The problem stared by the motor "oscillating". In one particular position, it would overshoot and cycle back - approximately 3/16" of an inch. No fault of any kind. No programming has been changed, nor have any speeds been changed.

I replaced the motor, drive controller , and amplifier module ($$$$$$) in attempts to fix the problem. None have fixed the problem although replacing the motor did help for a brief while. Eventually we slowed down the movement, so the oscillation did not stop the machine from running.
We have swapped out the drive controller and amplifier module several times and proven that it is only this particular motor that has the problem.

I've tried tuning this axis multiple times, and nothing seems to have an effect. Eventually, after several months of operation at a lower speed, now it will no longer run.

Now, it's newest quirk is it gives one or more feedback faults (MotFeedback,etc) and a "ManufacturerSpecificFault". It will fault out before ever even completing a homing operation.

Disconnecting the motor from the ballscrew - it still will fault out in free travel from a feedback fault!

So, at this point. Replaced feedback cable with a brand new one, no effect, same problem. Not one iota of improvement.
Cable shield ground is properly connected on only one side, drive side, so I presume it's not grounding related.

So what is it? I'm about to replace the motor a 3rd time with yet another known good motor... anyone have any thoughts? Do I just have extremely bad luck?

In case anyone is curious, this problem is resolved.

The velocity feedforward gains had to be multiplied by five in order to work. There was some minor noise on the line, even when properly grounded, which I guess was what was throwing everything off. This is not the "root cause" - there still is something underlying that has not been solved as this particular machine is identical to two others, and they all have the exact same hardware and servo parameters.

But whatever the reason, this fix has cost a tremendous amount of money and time. Finally an outside technician was able to solve the problem.

Hopefully someone out there may learn a little from this.