I have a '97 F150, the V6, with about 130,000 miles. When I drive for more than 45 minutes with the air conditioner on, the air flow is greatly reduced, to where hardly any air is coming out at all even at the highest fan setting. This happens on both AC settings. The only way to get it to work is to turn off the AC and run the blower at full-blast on the vent setting for several minutes.

 

sounds like low press. sw. if you can check the press. bring engine idle up. with blower set on low and if low side drops below about 24 psi. with out kicking out a/c clucth. then that sw is bad. it needs to turn off a/c clutch when it gets to a low press like that. or it freezes the evaporator. and shuts down all air flow. it becomes one big block of ice inside the plentiumn. you can change that very easy. its the sw mounted to the accumulator that has two wires going to it. if you can not check press. set on max a/c roll up all windows and blower on low. let run for about 10 15 min. until nice and cold in truck. bring idle up. about 2000rpm. a/c clutch should cyle on and off. if not. change that sw. its very easy. and not to much money. at any parts store. a/c is hard to diag when not looking at it. but i would bet its the low press sw. good luck.