[sierrap615]

if not as much volume of air is blowing after the car has been running for a while, but the settings are not changed and the sound of the blower motor does not change in volume, then something is most like physicly restricting the air flow. the only thing i can think of that would cause a restiction only on long runs is if the evaporatoir is iceing up, to test this, try to keep the A/C and defrost off on your next trip and use only the standard vents, if this is the cause, a tech will need to look into why it is iceing up. the only other idea i have if that does not work is see if there is any difference between fresh and recirc air. if there is, some leafs or someting may be caught in the cowl at the base of the windshield