I am happy -- overjoyed -- to report that the soldering trick worked. Thanks to all who kept urging me to "resolder the board even if it looks good." (Would that I had the time to solder one pad at a time and then test -- I don't know which one fixed it.) Actually, it probably doesn't matter, just that it now works. We'll see for how long this fix is good.

Note for thos comparing their problem to mine: this is a 1985 300D Turbo (123 chassis). My cc would (sometimes) engage and operate for a minute or two, then stop. At other times, it would fail to work entirely. Once, it accelerated with no end in sight, until I shut it off (at least that part worked!).

Some comments:
1. I found it easier to remove the board from the aluminum can than remove the can from under the dash. The screws, especially the rear one, are just too hard to get out (how did M-B get them in???). I carefully unbent the can where it was bent in against the board's mounting thingy, unplugged the wiring harness, and slid the board down and out. Carefully.
2. The board was made by VDO and wasn't as well designed as it could have been. The holes are not plated through -- this makes soldering harder and less reliable, as there's less surface for the solder to attach to.
3. The board was dipped in some kind of shellac or varnish, probably as a moisture barrier. While that may have been a good idea, it makes resoldering all that much harder, as you've got to burn through the coating, which also interferes with the soldering process. I added solder to almost every joint, more for the rosin (flux) that it contained than for the extra metal.
4. The pin structure for the connector is composed of multiple riveted brass components. On my unit, the brass was rather corroded; I'm surprised that there wasn't a poor contact there somewhere. I decided not to try soldering those joints because I don't know how soft the plastic composing the mounting base is.

One nice thing about this kind of problem is that if it recurrs, the same treatment will work as well as it did the first time. I hope I won't ever have to do that job again but at least I know how, now.

Jeremy in Santa Rosa, California

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970