Mark, thanks for the suggestion.
The air flow appears to be decreasing gradually until it comes to a complete stop and then it gives no air from any vent, even if I change the speeds, heat, or dash/defrost/bottom outlet settings. I have always had this problem when the AC is on, except one one winter it happened when the heat was on... It was a stormy day, icy all around, and perhaps I must have one of those heat/vent settings (e.g. defrost) where AC runs along with heat automatically in the background to remove moisure.
Anyhow, I like your suggestion and will try it out and let you know.