heating problem in an 2001 cougar

I have a 2001 mercury cougar with a 2.0 liter 4 cylindar engine and the heater does not work right. When the outside temperature is around 35 degree's or above the heater blows warm air. If the outside temerature is around 30 degree's or below it will only blow cold air. Iv changed the thermostat and it didnt help any. I was wondering if any one could tell me whats wrong with it? or how to fix this problem?

I have a 2001 mercury cougar with a 2.0 liter 4 cylindar engine and the heater does not work right. When the outside temperature is around 35 degree's or above the heater blows warm air. If the outside temerature is around 30 degree's or below it will only blow cold air. Iv changed the thermostat and it didnt help any. I was wondering if any one could tell me whats wrong with it? or how to fix this problem?
could be the blend door actuator.

What temp thermostat are you using? Are both hoses going to the heater core hot when the car is up to running temp? Is the blend door sticking?