I have to admit I had already taken the car to a shop by the time the advice rolled in, so I printed the photo and description and took it to him, hence the delay. He claimed to know about the fuel relay but hadn't checked it yet, but in any case that is what it turned out to be. The relay is not bypassed and the car is running great (for a 350!) so I hope this thread can help others diagnose what may appear to be turbo or IP problems.

Thanksgiving in the form of a Benz that exceeds 45 mph - thanks to all of you.
Paul