2000 Freightliner Century Power steering intermittent

As mentioned in previous posting I replaced the steering box and steering pump. Flushed out the system , new filter and no air. Have the front of the truck off the ground and have turned the steering wheel from stop to stop to bleed all the air out of system . when the truck sets for a while and I start it up the steering seems to be ok but as soon as I turn the wheel from full stop to full stop then the steering gets stiff again even when the front is off the ground. The front suspension is all new and I just greased it.
Vic

 

There is no air in the system. When the truck has sat for a a few hours. I can go out open the hood with the truck not running and manually turn the steering shaft with my hands. There is free movement to turn the wheels. When I start the truck , The steering seems ok but with just after a few minutes of running the steering gets stiff like no power steering at all. I shut off the truck. Go back outside to the steering box and try to turn the steering at the shaft , It is real stiff like there is some type of hydraulic lock.
Let it set for a few hours and repeat the same senario
It seems like something is hydraulically locking the steering.