I have had the same prob.. in my case it was the servo that controls the air flow... vent,floor,defrost,etc.. my mechanic removed the servo and replaced it for $125.. never happened again
If it is the servo pop the panel off from under the glovebox, Remove the glovebox assy.With the key on engine off you can press the mode button and watch the servo move, Turn the key completely off.Remove the 3 screws and plug to take the assy. out. Do not attempt to rotate the arm on the end of the servo( it engages with a worm gear and will break the gears )The servo is a snap together unit. Carefully take it apart with the arm side down so the gears don't fall out.Inside you will see multiple whiper fingersthat make contact with a gear, Carefully clean these with alcohol and a cotton swab.Redistribute the contact grease with a clean finger, Carefully snap the unit back together , Clean the copper contacts and put everything back together. Lexus wants 2 hrs. labor for this.