Ford: A/C repair, vacuum gauge, ford bronco

Question
I have a 1996 Ford Bronco, full size with a 5.8.  With the air conditioner on high and recirculating it will come out the vents but then start coming out the defrost vents for no reason.  Changing the controls doesn't help but after a few minutes it will correct itself.  I was thinking a blend air door problem but I don't hear the door opening or closing nor do I hear a vacuum leak.  Any help would be greatly appreciated.

Answer
it could be a leak in the door actuator, or it could be the controls itself. this type of problem is one of those frustrating ones that are hard to diagnose and require patience and persistence. check all the vacuum lines for crack or leaks, you may not necessarily hear it, a vacuum gauge pump helps with this type of problem.