Well, this is 50,000 miles later and I finally figured out that the problem was in the cruise control amplifier. To re-cap, it worked intermittently, hardly ever in the winter time and almost no problem in 90 degree weather after the car was parked out in the sun. Two years ago I had bought another used amplifier that did not work at all. When I told my service guru the other day what happened, he laughed and said that on the replacement you had to disconnect the second ground! And sure enough, he did that and now the cruise control engages every time you try it. The replacement came out of a 1986 300D and sadly, it has other problems, does not like to stay engaged for very long and you have to hook it up again regularly. But at least now I know where the problem was. :-)