Thanks BenzMac. That was the problem.
I read your reply last night and, as luck would have it, I received one your competitor's (name withheld...) parts catalog yesterday. I'd ordered it just for the vacuum diagrams I'd been told it contained. I opened it up to the transmission vacuum diagram, compared it to my car's and voila! The setup was wrong!!!
I know it was still as it was when I bought the car. I'd replaced some parts and moved stuff around but I'd made a drawing of the setup and always put it back as it was. I guess the previous owner - a shop - wasn't as careful.
1982 300D - <A HREF="http://members.xoom.com/nicodore/