youve probably got a vacuum leak somewhere. the air flowing at first is keeping the air dam up/down for the middle vents, blocking them off. so when the air stops, they fall/ open. kinda like a shop vac when you knock it over, youve got to turn it off then back on to continue vacuuming.
this is just my theory.
98 TJ sport 4.0L I6 356,000 miles.
No Engine, axle, tranny, or t-case rebuilds.
Check out my 5.3 swap in progress here!