No air from heater vents, random problem

We have a 95 Lumina APV with the 3.8L. Sometimes air stops blowing out of the vents, and after a while will start working again. I can hear the blower motor running, and can change fan speeds, but when it stops blowing air, changing the vents (heat, bi-lev, vent, defrost, etc) has no effect. This occurs randomly, and the air flow stops suddenly. It acts like maybe a flapper in the duct system is blocking air flow, or something. After a while it will start working again for no apparent reason.

Any suggestions?

The vent control system is vacuum activated and very sensitive to changes in vacuum level. Climbing a long hill depletes vacuum and it will generally dump the top panel vents. They come back once on level or downhill road.
Any loss in vacuum tubing will increase this behaviour.
VEnt control solenoids are below and left of glove box but check for leaks anywhere in the vacuum system, power brake, cruise control...etc.

Thanks for the reply.

I've been investigating this problem further since I posted, with the same idea as your suggestion. The problem does not seem to occur during any low vacuum conditions, necessarily. When it did occur, I tried downshifting, and using engine braking to supply maximum vacuum to the system; this did not cause the vents to come back on.

I did discover that when the top vents stop blowing, the air comes out the defroster, rather than being completely blocked off as I originally thought.

I think I've identified the problem as the little electric/vaccuum control unit mounted to the blower housing below the glove box. I removed the kick panel and drove around until the vents stopped working. At that point, I reached down and tapped that box with a wrench, and the vents started working again immediately. I suspect the problem may be a bad electrical connection in the box, as I can't find any vacuum leaks around it. Unfortunately, it looks like you have to pull the dash to get the mounting screws out of this little box.