97 Climate Control

Did a quick review, and didn't see this covered. After 20 mins. or so of driving, or parked idling, my a/c stops working! I don't think it is inop., it seems like the doors or ductwork or something is closing? If you turn up the fan you can hear it running, but nothing is coming out(very little) of the vents. Dealer has replaced the temp sensor, and that didn't fix it. Going back in next week to try again, but any help here would be appreciated. Mileage is 36,000. Although it's cool here now(CT) I don't want to loose my defrost capability if the air is being blocked somehow.

Thanks for reading.

Phil