As brewski suggested you may want to get a pressure guage and use that to try and narrow down where the pressure loss is originating.
But based on your description and symptoms, your not wrong about the priority valve suspicion.
You might just be loosing system pressure due to the pump being worn out and when it gets hot it just stops building pressure and bypasses. (hence why you need to get a pressure guage and see whats happening).
According to the breakdown in the hydraulic manual this pump has 2 outputs, priority flow and steering output, there is a priority valve on it. So you are probably in the right place, just get a pressure guage and measure the pressure on the pump output, when you loose functionality i think you will see the pressure drop, probably when it warms up.
Once you verify this being the problem, get a new pump and it should work correctly again :o)