Stiff foot throttle

Hi 1983 chev silver 454 enagine 700r4 transmission
truck complely rebuilt.

the foot throttle keeps getting stiffer and stiffer to depress. Each time I take it in they do something to the valve body and it becomes very free like it should be. After a long trip like 800 km the throttle pedeal becomes stiff and over a period of time it becomes almost impossiable to depress it.

I have a cooler mounted in front of the rad. I had the cooler lines by pass the rad transmission cooler and go straight to the trans cooler it still did the same thing. So I put it back the way the book calls for.

I asked the mechanic what he did and he said cleaned out the throttle valve in the transmission.

the transmission is a rebuilt and the shift points are correct ass well as the pressires according to the service manual from GM.

this has been going on for 4 years now. The old transmission never did that.
any ideas?

hank