This will be some sort of vacuum problem..... climate control 'mode' selection uses engine vacuum to operate the doors. The system defaults to the defrost setting when vacuum bleeds off.
Take note of when the problem occurs...... when accelerating or ascending grades would most likely be the one way check valve - usually located near the firewall on the right hand (passenger) side. If the problem occurs when going over rough roads, suspect a loose vacuum connection.
HTH.
-- Jim Warman snipped-for-privacy@telusplanet.net
Our 97 Explorer has developed a problem. While running the A/C the air
> control will switch from the dash vents to the defroster vents then back to
> the dash vents. This cycle repeats periodically and frequently. The control
> dial is set to Max A/C or A/C and happens without any driver interaction. >
> Is there a faulty relay that is the most culprit in this possession? >
>