Weird Ventilation Fan Problem

[max123]

Maybe someone else has dealt with this before, because I'd rather not go on a fish expedition under my dash.

Over the winter I noticed that my ventilation fan was blowing cold on really cold days.  I figured it was the temperature control valve.  Fine - but I recently noticed one other quirk.  If I have the fan on 4 and the temp guage all the way to cold I get tons of air.  Turn the temp gauge to hot and something restrict the flow alot.  It's almost like the vent gets blocked.

Any ideas - I thought that the temp control gauge wouldn't have any connection with airflow?

[longtallsally]

Perhaps something is clogged in the heater core or the flap for the heater valve?  Have you verified that the heater core and valve are working?

[max123]

Haven't pulled it apart yet - was hoping to get some ideas first.