BMW Repair: 99 BMW 323i A/C Blows Hot Air, quality control test, bmw 323i

Question
Last year (and presently), after the A/C runs properly for a while, it starts to blow warm air.  The air is no longer conditioned, and it seems as if the system is blowing heated air from the engine.  It is very intermittent.  For example, I drive 2 miles to the store and it ran fine.  On the return trip home, it began blowing heated air in less than 1.5 miles.  Other times it may take 15 or more minutes before it changes.  It always works properly when we first turn the car on. When it changes, the outside vents blow cooler (though not conditioned air) than the middle vents.  Last year and last week, the BMW Dealer removed and replaced the Freon.  However, that had no effect.  They cannot identify the problem. It's been over a year, it's hot, and we have small kids to transport.  What is the problem?  What's the estimated amount of hours to correct it.
THANK YOU!!

Answer
Did the Evacuate and Recharge work for a little while or immediately or not at all? Take the car back to the dealer and tell them their repairs were ineffective and you would like to speak to the service manager about discounting the work or have the car checked by the shop foreman free of charge. I don't know how much money was spent or if your system was in fact low on charge.They should have caught it in a Quality Control test drive. See if they will pay for your rental car. Squeeky wheel gets the grease, be assertive but not an jerk. They still have to want to do business with you. You will want to speak with The Shop foreman and go on a drive with him/her. I would look at a Water valve letting hot coolant into the heater core (not supposed to when A/C settings are set to cold). Have them check Auxiliary electric fan operation. It seems that it works well when engine is cold and not as well when hot. Be very specific with the Shopforman. Hope this helps.
-Evan