Hi, I have a 2001 GMC Yukon XL. I'm having problem with the a/c. When I turn it on it will work fine at first for the first 30-45 minutes of use, the a/c blows cold air. But after that the a/c does not blow cold air anymore. It'll just blow warm air. I already checked the compressor, new ac relay, R134 pressure is ok,.. any idea anyone? what could cause the a/c to stop producing cold air? thanks