89 Town Car AC dampers

IN my 89 towncar, the doors inside the AC system act crazy. on start up and hwy driving it runs normal. but after some time of city driving the damper that controls how much air comes out slowly closes and the fan is running, but no air out any vent, and the control is on panel

i can move it to floor, and have full air, or move it to defrost and have full air, and back to panel, and all the doors shut and the fan runs, moving no air. anyone have any suggestions?