Heater problem with my 2004 Mountaineer

[motc777]

My AC works fine. That's not the issue.
When I turn on the heat, no clunk noise behind the dash, nor any clicking sound.

What happens is that I turn on the heater, and it takes awhile, but it will blow hot so long as the car is moving. The moment we stop, it gradually cools down and then blows nothing but cool air.

Any ideas?

 

[Number4]

Almost sounds like a vacuum issue controlling one of the blend doors. More vacuum while driving and less while stopped? Causing the door to shift from Heat to Ac? Sounds plausible, but don't know.