I have this same problem and went through the ringer with the dealer. I ended up bring it to Blanding Automotive -- a Volvo specialist in Jacksonville, FL who go out of their way to help. They put an iodine through the system and had me drive around for about 1 month then return for inspection. Turns out there is a compressor leak. They also pointed out to me that there is a safety bulletin -- search NTSB for recalls and service bulletins stating that the compressor may need servicing and/or replacement. If you have an extended warranty this is covered, however, otherwise, push and pull the dealer to work with you. Best of luck.