2002 Chevy Trailblazer air condtioning cuts out while drivi

I have a 2002 trailblazer, my a/c works great when I drive at about 1500 rpms (city driving), when I cruise down the highway at 2000 rpms the a/c cuts out every once in a whille. It blows warm for about 1 min then the pump kicks back on and it gets cold again. It will do this about 5 times in an hour. I had the a/c system checked out by a profesional and he replaced the pressure switch on the a/c pump, also recovered the system and put 1.8lbs of R134a in, and now this is what it is doing, I would perfer not to take it to him again. Please give me some advice on what it might be