This summer when running the A/C on the normal switch setting, the air flow jumps back and forth between the defrost ducts and the panel ducts.

Now, when it's on heat Floor/Panel setting, the A/C unit runs (can feel the pull on the engine) and the heat air flow randomly flip flops between the defrost ducts and panel.

The only setting that seems to work normally is when the control is on Heat and the air flow position is set to Floor.

Is this a blend door issue, or the dash A/C heat control unit?

Thanks, Sam

P.S. This is a '97 F-150 xcab w/125K miles.