2008 Navigator Climate Control

My Navigator's automatic climate control will periodically go from A/C to heat. This happens when the outside temperature reading will start dropping suddenly from a temperature around 80-90 degrees to 50 degrees and below, even though the ambient temperature stays at 80-90 degrees. I have ran a diagnosis of the climate control, using the control panel, and it shows no errors. Before I take it to a dealer and spend an outrageous price for a diagnosis; I was wondering if anyone has had this issue before, and/or has an idea how to repair this problem.