At highway speed the cooling is insufficient, stop at a signal light and the AC gets cold. I thought it would work the opposite. Freon (whatever) added and brought up to specs, dye added to check for leeks. No leaks show and no difference in operation. Any ideas?

 

How is your recirc system working? If it's set to outside air, then that could explain hot air entering the cab at speed. Even if you have it set to recirc, the door could be stuck open (common problem in our model years).