Your description would suggest that you have found the fault but not the cause, if replacing the orbital valve cured the fault but it came back. I have known the coupling in the tandem pump between the main pump and the steering pump break up sending small pieces of debris into the orbital valve causing all manor of strange symptoms, it may be worth checking this. Also it may be worth resealing the ram as over time the main pressure seal wears in the area of most use, again giving similar symptoms.