Thanks for all of the good information. I tried Randy's (and Arthur's offline) thoughts first and sprayed some penetrating oil into the two swivel joints between the firewall and the booster. The symptom went away. I am not so cavalier as to say that the problem is solved but no symptoms is a major step in that direction. If the symptom reappears, I will explore further.

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Jim Villers
190SL, 230SL 5-speed, 95 E320 Wagon, 01 E320 Wagon, MGB, Boxster 'S', 190SL "Barn Find"