2003 Deville Heater not working properly!!!
 

I have a problem with my 2003 Deville heater... When I put the heater on the hottest setting (I think 80 degrees) it blows nice and hot no problem, but when I turn it down it still blows very hot even down to 61 degrees until I get to the lowest setting which is 60, then it blows ice cold... If I turn it up 1 degree to 61 degrees it blows red hot again... Any advise would be very much appreciated, please help!!!