When it stops cooling on a long trip, try switching to the economy position on the pushbuttons. Wait five minutes, then switch the a/c back on. If it cools, most likely your evaporator has frozen up. It's too cold, so the condensation removed from the air flowing through the car freezes on the evaporator rather than stays liquid and drains out the bottom of the car. This behavior only happens on long trips for two reasons. That's the only time you operate the car for a long enough time to coat the evaporator with ice and 2) the a/c can cool better on the highway, so that's the only time the evaporator is likely to be cold enough to actually freeze the condensate.

Your car has an evaporator temperature sensor which is supposed to cycle the compressor off when the evaporator gets too cold. It may be defective, or the copper tube it uses to sense evaporator temperature may not be in contact with the evaporator.