HVAC stops blowing



I drive a 2000 Nissan Altima and when I am driving for long periods of time (over 30 minutes) the HVAC stops blowing. You can still hear it working (in fact it sounds louder) but only a small fraction of the air makes it out of the vent. This effects floor, defrost, and dashboard vents evenly and A/C and heat. I think it must be a value that is failing. Oh, and the problem seems to correct itself if the car is off for more than an hour, but stopping for gas doesn’t correct anything.