HVAC vacuum diagrams

Blend door controls the air temperature. Mode doors control where the air goes.
If you cannot select hot/cold it is the first one, If you cannot select floor/dash/defrost it is the second.

In the case of an F150 you have a total of four vacuum actuators. One controls the fresh air/recirculate, one controls temperature and two are mode control.

All can be tested with a hand held vacuum pump at the back of the control panel.

I do notice air flow through the dash board defroster vents and air flow through the vents down near the gas pedal when the switch is set on defrost or floor/defrost.

That sounds like the panel/defrost door is not operating properly. It is on the top left of the image. Connector 6 red on the control head. It could be a bad vacuum line or bad control itself. As you say it is intermittent, I would bet it is a bad line to the control. Sometimes it gets vacuum and other times it does not.
The control should be visible if you look up at the heater box from under the drivers side of the dash. The line should be easy to spot as well.