On several occasions the windshield has unexpectedly fogged up while driving. this severely reduces visibility. the windshield takes several minutes to clear with the environmental controls set to high heat, high fan and full defrost. i have experienced this problem anywhere from 5 to 45 minutes after driving the vehicle. on every occasion it has happened when the outside temperature dropped to 34f and the air-conditioning shut down. we have experienced this problem on our other fleet vehicles of the same make model and year. *ln