Heat/AC on my 1980 450SL would not operate at all. My mechanic bypassed the "main" vacuum switch with a toggle switch mounted under the dash and everything started working fine. He indicated that sometimes the older the car the greater the tendency for the vacuum level to flucuate causing problems. This solution was ok with me since it was cheaper and could be reversed and fixed according to the book later if I wanted. Hope this may give you some options.