I am having an intermittent A/C issue. If I turn off A/C before I turn off the car, then restart the A/C after the car has been running a couple minutes, the air works fine. If I leave the A/C on and turn off the car, then turn the car with the A/C already on, it blows hot only and takes up to 20 minutes to start to get cold. Is there anything I should check or anything about to fail? Any help would be greatly appreciated.