basic a/c diag. if you do not have/system not equipped with evap temp sensor to cut off compressor when system is cold, system will continue to operate thus freezing the "h" valve causing it to freeze over and not cool. next time it happens, shut a/c off for 90-120 seconds and turn back on, if it cools again, "h" valve is freezing over from over work. then it would need a compressor cycling system of some kind.