Climate Control????

I own a 2001 Jimmy SLT with 42k on it. The problem I'm having is my automatic climate control, if I set the temp at 82 I get more heat then any one could ask for, but if I lower it to , let's say 78, all I get is cold air. It's been to the dealer several times and they cannot locate a problem. Does anyone have a clue as what this might be. I realize the unit has a s*****t load of sensors and controllers, but I also believe I'm not the only one who has ever had this problem.

Thanks for any response
DJR