Steering stiff on rack & pinion after driving

[1FUN64]

I have rack and pinion on my 64 shorty and when I first get in the truck and start driving it works great, I can turn the wheels normally sitting at a stop. Once I drive the truck for a for a while and everything is warmed up if I come to a complete stop I can't turn the front wheels. If I am moving slow they will turn but the steering wheel is pretty heavy, driving at normal speeds it feels pretty normal.

The rack is from an early 90's Tbird and the PS pump is a GM from a 90 1/2 ton Suburban.

The supplier of the rack sent me another one thinking it was a valve sticking but the same issue with the new rack. I am still working with him also just know this is a good place for valuable input.

I am going to "T" in a pressure gauge to see if it is falling off for some reason.

Thanks for any input guys.