I joined a vessel with a vehicle that had some really odd problems with its Orion. The wrist rotate, jaw and forearm functions seemed to have a mind of their own.
We tried all the obvious stuff. It then became apparent that there had been a problem during mob and the vehicle hydraulics had been overpressurised. Somehow this had transferred to the arm and blown a series of seals in the forearm manifold.
The forearm has a stack of (I think) 9 seals and back up rings that act like a hydraulic slipring (rotary valve I guess) and allow the hydraulic fluid to pass through the rotating joint. Five of these seals had been distorted and allowing fluid to take the line of least resistance and move whatever it could.
Maybe you have a similar situation.
It really is a workshop job as it is not much fun on the back end of a ship.
Andy