I have a 2003 EXT. Temp in MN is cold right now and most of the time lately when trying to adjust the temp up or down the heat control knobs don't work very well. The best way I can explain it is if the control temp is 70 and I turn the knob up to 75 the display will maybe will jump up to 71 and then down to 65? Both drives and passengers seem to acted the same. Once the inside of the cab warms up it seem to work better. Anyone experienced this? any fixes other then replacing the heater control unit?