I have a question on nomenclature. What is the name of the door that the temperature dial controls on an '08 XLT with manual HVAC controls? Is it the temp door or blend door?

Here is why I'm asking...During steady state driving (highway at 75 MPH) I normally run in panel vent mode. The coolant temp is at 192 degrees and I'm quite certain that the t'stat is fully open. I keep the temp dial at full heat, panel vent mode and the blower at M2 (second notch from the highest setting). Once I feel comfortable I turn the temp dial down to somewhere between the 12:00 position and the 9:00 position. The air temperature will fluctuate RANDOMLY between really cold and really hot. I can even hear a fluctuation of the air flow as the door (temp or blend door) is moving. When I say really hot I'm saying it feels like it is on full heat like the dial is turned all the way to the right.

I'm thinking that it is the door actuator but I don't know what the proper term is. The truck is still under warranty with only 12,000 miles on it so I'm not worried about how to get it fixed. I'm asking because I want to explain it clearly to the dealership service people when I take it in.

I have no mods except for my CB and my Edge. I can't imagine that these would have any effect on the panel temps but what do I know.

Anyone have any ideas? My searches here on the board didn't really reveal anything that sounded quite like my problem.

Thanks.