edit (changed Model Year to 2003 misread the Vin XD
This is one of the strangest things I have ever encountered. when i start my car the climate control system works perfectly and it never has any issues driving around town. however for the past year or so every time i take a long drive over an hour and a half the force of the air coming out of the vents starts to diminish to the point where the only circulation i am getting is from the outside vent. the strange thing is i can hear the fan blowing behind the dashboard and when i hold my hand to the vent the air coming out if very cold / hot depending on where i set the dial so it doesent appear to be an issue with the AC or heater core .
I am thinking there might be some type of obstruction but that doesent explain why the symptoms occur like clockwork on long drives.