Thanks Steve!

Your reply suggested that the source of my problem was either: one or more of the vacuum servos, a bad switchover valve, or a vacuum leak.

Well, the problem turned out to be a vacuum leak at the one of the vacuum lines that goes from the engine compartment (near the instrument cluster) to the climate control area. Once this vacuum leak was fixed, the system operated normally again!

Thank you for saving my technician and I alot of time!

Ed