I have a 1998 Dodge Ram 1500 w/ the 5.2 v8. When accelerating at approximately above 50 mph the AC stops coming out the dash vents and only goes through the top vents by the windshield. the air wont come back through the dash vent until i let off the acceleration, & it makes a noise like a door flap or something shutting then the air comes back, & again when accelerating it stops. Can anyone tell me what it could possibly be?