The AC unit in my 2004 MGM (172K miles) won't blow cold until after the car has been running for about 40 minutes. That's when the compressor engages. After I shut the car off, some times the compressor kicks in and some times it doesn't. I know the system has freon because it blows nice cold air. What could it be? Could it be be the compressor clutch? Any comments would be appreciated.