If the hyd's started slow I would say the FOOT Valve that holds back main valve center flow (NFC) to keep pumps at minimum angle was full of debris. But since you say the hyd's, all functions, start good then slow down I feel a calibration of the PRV solenoid is in order. The NFC,negative flow control, pressure tells the pumps you intend to do nothing so they should pump nothing at this time. When you move a lever and the spool in the control valve moves it pinches off the center flow of oil, the oil building pressure at the Foot Valve stops coming and the pressure falls, asnthismpressure falls the pump or pumps stroke up to max to do work. This is happening on your machine. And when it does the engine is loaded up and RPM's drop. As this happens the ECM tells the PRV solenoid to adjust the Powershift pressure to stroke back the pumps and allow the engine speed to recover to Target RPM. I think the PRV is over doing it.
Later Bob
