Hello, I have a 91 240 sedan. My a/c has been replaced since last year. The compressor, receiver dryer, even the electrical switch is new. We drove about 250-300 miles today in the Florida heat on the interstate at around 70-80mph. After maybe 15 minutes of nice cold air it seemed to stop blowing cold, I shut off the a/c switch, then it would get cold again, even colder than before, then I would put the a/c switch back on and we were good again for 15-20 mins. usually we do not drive this car very many miles at a time Any idea of what is going on? thanks!