Could be the engine warming up over a threshold, Im not certain on the 3rd gen trucks but there are a good amount of cars that will turn off the AC if its starting to heat up over the fan threshold.
There could be a few more issues. The ac system could be polluted, if it has a leak water gets in there and really reduces the cooling capacity, so it required more pump speed to get it down to temp, a way to test this is if its in park, and idle is it cold, does it get colder when you hold it at 2k rpms? This usually indicates pollution in the system or the compressor is failing.
Last thing off the top of my head would be the fans are not working properly, its an engine driven fan correct? The clutch might not be engaged fully and its not pulling enough air over the condense to again cool the coolant.