my vents (blend door) close on long road trips and reopen after i slow it down for awhile, like in traffic.

 

My 94 ram 1500 did that with the 5.2L was from a vav leak on one of the vac hose going to the ac controls. vac drops at hiway speed and it would swich to reg ac if I would let off the gas it would swich back to max ac after vac line was fixed worked great.

 

my 97 did that trick. 03 is electric control.