Heater fan looses airflow??

After I drive my 99 for 20 min or so the airflow out of the vents or defrost or whatever setting I have stops. The fan is still running but it directing the air somewhere else. If I switch the air selector from outside to inside I can get the airflow back for awhile, then it goes away again. I am assuming the the door that selects the inside/outside air is opening halfway and somehow the airflow is getting sent outside the van. I am also assuming that the problem lies somewhere in the vacuum harness. I am wondering if anyone else has run into a problem similar to this?

Have you tried changing the Cabin Airfilter? I found that killed my airflow after wondering and curiously finding the filters access threw the glove compartment.