I'm not a A/C person, but do you know if your 'Burb has a cabin filter? If it has one has it been changed. Years ago I changed my filter in my'96 'Burb and that seemed to help with the cooling. I wasn't having any issues with the A/C but I changed it as a maitanance item and it did seem to help cool the 'Burb quicker. just a thought.