Blower quits blowing AC when driving sometimes

Thanks for the tip. I figured out today that the system is freezing itself out. I shut off the air conditioner and it started to blow harder in a couple of minutes. I am going to take it in and get it topped off and have the system checked out. I have had the truck ~4 years now and I guess it is time. I wish I had figured it out before tearing the dash all apart checking the recirculation door.