Air flow problem with heater/defrost/ac

I have a 2000 Dodge Neon with 160,000 miles. For the past few years I have had an on and off problem with the air flow when using the heater/defrost/AC. It works fine for about 30-45 minutes, then the air flow fades away. The fan is still running and the air (what little you can feel) is still either hot or cold. Its like its being blocked, but will sometimes come back, sometimes not. My dealer cannot duplicate the problem but did replace the fan motor. This is one of those problems that only happens when you need the HVAC the most. Does anyone have any ideas? What do I need to check and or replace? Thanks.

Greg

i think the little doors that allow the air to flow through the heater/air conditioner are controlled by vacuum diaphrams. So I would look for a leaking vacuum line serving the control unit, or between the control unit and the heater/a.c. This is of course a hard location to get to - under the dash, or at the back of the eng. compartment.
If there is no vacuum, the default is supposed to be that the defroster gets the air instead.