Update:

Checked fuel pressure, values are right on specification per All Data.
3.4 with vac to regulator
4.0 without vac to regulator
no leakdown noted after shutdown

Replaced O2 sensor even though no codes present. (Bosch and MB both recommend replacing at 60K miles, mine had 100K miles)

Symptom still present.

I believe I have reached the limit of my DIY abilities.
I would be happy to pay a shop with testing equipment to diagnose/fix this problem. Before I engage the help of a professional, I would like to have some idea of what the next step might be. Any information from the pros would be appreciated.

J. M. van Swaay