I am also having this same exact problem. I have also noticed that I can pull over and turn the ac on and then off and it will fix itself. I figured if the evaporator was freezing over I would be able to run the heater for a few minutes and it would fix the problem but it does not.
I asked a mechanic and he told me it sounded like I have a vacuum leak because apparently a car has a lot of vacuum when it is sitting still but not so much when going down the interstate. I have looked all around and don't see any vacuum line going to the ac.. can anyone confirm whether or not a vacuum line goes to the ac?
Also, I have seen the drain tube coming out of the firewall. How is it recommended to unplug this? Draino.. just kidding. Is there something I can spray into it to unclog it?
I have noticed exceptionally large pools of water in the garage after driving it....