I have a BMW 320i 2008. When I turn on the hot air and Automatic recycle, after a point, it blows cold air for 5-10 mins, then back to hot and so on. This doesn't occur when I put the recycle on. Any clues?
Google for "bmw e320i heater control valve" when the air con regulates the temperature, it opens and closes the valve. If the valve sticks, it may just give cold or hot air. When this happens, you may be able to check the voltage at the valve connector to see if it is getting the correct input. There may be connection problems also. Usually cycling between maximum hot and cold few times can release it and return to normal (Which would point to a faulty valve).
If this happens often you can also test it by setting highest heat setting and regulate temperature with fan speed. If all of a sudden it gives cold air, it may be the connections. Because in highest heat setting valve should stay in same position.