Already suggested that in a previous thread to him:
http://www.peachparts.com/shopforum/showthread.php?t=287087
He said "no change" so I would be tempted to say it almost has to be the duovalve, unless you have another suggestion. Of course I mentioned that the engine coolant has to be not low and the engine has to be warming up. Other than that I don't know what else it could be except a plugged heater core.