2002 Park Avenue Heat Control

In order to get the temp reduced in the cab of my PA, I have to lower the temp setting to at least 65 degrees before it will begin to decrease the output temp. Likewise, If it is cold inside, I have to bump it up to 75 degrees before it will begin to push out warm air. I have tried "auto", but it does not help. As a result, it tends to go from too hot to too cold and then back up again. Does anyone have any ideas about this? Are the controls possibly faulty?