One thing to check is the flap that regulates the recirculation of air. Its easy to do. Take out your glove box and right behind it are two vacuum valves that control the flap. Next set ac to the max cold setting (min temp) and watch the flap. It should open and not flap around (no pun intended). I have seen this once and heard about it a couple of times. The tip off is that it works sometimes and not all the time. Just one easy thing to check.

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Mark B

82 300SD 225k
81 300D dead at 207k