1997 Mercury Mountaineer No air blowing from dash board ven

I have an intermittant problem with air blowing from dashboard vents. Sometimes it works blowing hot air for heat and cool air for a/c. So I know the blower works ok. Most times nothing from vents or defrost or floor vents, when I turn selector to desired location. Rear controls and vents work fine. Weeks go by and it does nothing. Then one day I get in start the vehicle and I have air blowing thru front dash vents. I can't figure this out, can you help me?