I had similar symptoms and it seemed like the AC had a mind of it's own. Resoldering the CCU fixed the problem. I assume it was not sending the 12V to the monovalve part of the time when it should have.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html