Presumably you are not trying to lift anything heavier that 750-800 pounds.
Are you seeing anything suspicious through the RH access hole when you try to raise a load on the three point?
Do you have full travel on the control valve(~ 1 inch fore and aft total movement)?
If there are no oil leaks in the supply to the lift cylinder or at the pump housing (leaking side plate gaskets/seals) and the control valve is moving through it"s full range, then I would expect a problem internally in the pump. Check valves?