I would presume then it's a wet pack 'hydraulic shuttle'. If it is, the clamping and unclamping pressure is out of speck or the proportioning valve itself is faulty. That happens, they are (valve) is real sensitive to any dirt in it.
First thing I'd do is a complete fluid and filter change and don't use cheap TSC hydraulic fluid either. Use a quality (and mid priced fluid) and quality CNH filters too. That may cure the issue. If not, you need to check the clamp and unclamp pressure via the port on the valve with a liquid fill 4000 psi pressure gauge. If it's within specs (which will be in the workshop manual) then it's time to pull the valve and replace it. It's external to the transmission so not a big deal, least on mine it is.