Drivers side blowing hot air with A/Con and passenger blowing cold.

I have had precisely the same problem with my 2012 Edge. It does this intermittently-not always-- and without warning. It makes summer driving unpredictably intolerable. Have complained to dealer but it has not malfunctioned while at the dealer and it says that it cannot diagnose problem unless it malfunctions while at the dealership. Please let us know if you figured it out

MY 2013 egdge is blowing cold air on defrosters on drivers side while warm air on passengersn side