Something to do with the thermostat or the fan on the front of the truck would be my guess.
If it takes a while before you start getting cold air again, it is possible that the evaporator is freezing and obstructing airflow over the evap.
First of all make sure you have a clean cabin air filter. A dirty one can restrict airflow and cause icing of the coils if the humidity is high.If this is the problem I w/the a/c what needs to be done?
It does not take a long time for it to blow cold. I start the truck up & it instantly cold. When I get on the frwy @ about 50 mph + I lose the cold air. When I slow down to exit it comes right back.