Ok, the door actuators that control the air direction doors are vacuum controlled. A vacuum leak either in the engine vacuum hoses or from the hose going to the vacuum reservoir or the hvac system will cause erratic idle and vent direction issues.
The vacuum reservoir is mounted in the engine compartment on the right cowl side panel near the speed control servo and below the right headlamp. I need you to check it first for disconnected or broken vacuum lines, as well as cracks on the tank itself.
Next, with engine idling, use carb cleaner and spray every vacuum hose in the engine compartment one at a time, including the pcv valve hose and brake booster hose. If the idle smooths out when one is sprayed, that is your cuprit and you will need to replace it.
If ok, then we will have to dig into the dash to check the vacuum hoses behind it going to the door actuators
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