heater problems in my windstar

My vans heater stopped its airflow twice one hour into our trip on the weekend. Everthing else seems to work but the flow of air from the front and the rear heaters stops. Fans continue to work. It seems like it is slamming the door shut and stopping the air flow. It also seems to do with the ac controls. Once I stopped and use the ac control and get it to blow air then I can get the heater to work again. NEED HELP! :uhoh:

Thanks

model year?

The model is a 2001 sport!
Thanks