1997 Buick Park Avenue Blend door problem

I have a 1997 Buick Park avenue,with the regualr 3800 motor, no supercharger. The problem I am having with the A/C has been going on for a couple of years. When I set the climate control on full A/C, meaning 60 degrees, the passenger side vents blow full cold, and work well. The driver side blows a lukewarm mix of A/C and heat. When I first start the car and the engine is cold, the air coming out of the driver side outlets is pretty cool, although still not as cool as the passenger side. Once the car warms up, then the air gets warm. I think there is air coming from the heater core. In an attempt to repair this, I have replaced the dashboard control, the control module under the passenger side of the dash, and the blend door actuator for the driver side. After replacing the blend door motor, and still having the same symtoms, I left the blend door motor off, and turned the shaft with my hand. When fully clockwise I would get the full heat, when fully counterclockwise I get the lukewarm air. All I can think of at this moment is that something is wrong inside the box. Please help.
Abe.