AC unit problem

I recently purchased a 2005 Dodge Stratus only 16k miles. Its in perfect condition, except its Haunted. I turn the AC on and even through the dial is set for the air to blow out of the frontal vents and/or the floor vents, it blows out the windsheild defrost vent. Without touching the dial, it has a mind of its own (ie, Haunted) it changes from one setting to the other in regards to which vent the AC comes out. Usually I just let it blow out the defrost vent, except its 100 degrees in Pawleys Island SC right now and I need the frontal vents to work to cool the car. The AC is Ice Cold. Sometimes I jog the dial really fast back and forth from setting to setting and that trips it to blow out the frontal vent but some time it just sticks to the defrost vent. I’m not sure this is a factory defect or what? All the reviews I’ve read on this car says its very reliable. I bought it from an auction that only sells cars from Government Installations, and they buy the cars without any warranties to save money,and do all maintaining work them selfs and sell As Is. Any suggestions? My dad has purchased many low milage cars from this auction (which they have in every state) and has never had a problem. Any help would be greatly appreciated!

Help AC Unit Toasting my Taters in SC

That sounds like a vacuum problem. Most cars today use vacuum controls.

There certainly is a vacuum component retained in many HVAC systems but electric mode door motors are also very popular. Never had the pleasure to explore the Stratus.

A manual for this vehicle would be very helpful.