HVAC regulator

OK I'm having issue with my 04 Quad Cab. The HVAC is refusing to properly regulate temperature. You need to turn temperature dial completely to the cold position to get cold air then as you rotate the knob the temperature output correctly (more or less) rises in proportion to the knob setting. At about 1/2 way to hot you can hear the regulator door shift and the temperature output jumps to well over 100 degrees, maximum output at center vent is about 120 degrees coldest is about 55 degrees. To reduce the temperature you need to rotate the knob completely to the coldest setting you hear the door shift again then you can increase it to the desired level.

Dakota been in the shop 4 times had 2 head/control units installed 2 reprogram/calibrations nothing has corrected the problem. It's scheduled to go back into dealer on Monday for another try.
Has anyone had similar problem and what was done to correct it?
Any ideas what is causing problem and what should be done to correct it?

thanks.