A/C doors don't want to stay open/close

When my '94 RSW is idling or just driving around town, there's some part of the a/c up in the dash that sounds like it's trying to open/close a damper (???) and it just keeps cycling. It does this most of the time when the outdoor temp is close to what I'm trying to achieve in the cockpit. It doesn't do it when there are extreme temperature differences of I'm running down the highway. Any thoughts?

Thanks,
Ed

Your check valve may be inop. There are two main control flappers. One is for hot/cold air, and the other is for the dash vents/defrost/floor. A third one is at the intake for recirculating air. You could have a vacuum leak, or a vacuum motor may be failing.