waylifeshouldbe,

I experienced all what you described in the long post and replies.

I changed along list of items on my 260E, EU 1988, including resoldering of Fuel pump relay (see my post "loss of power while running).
I am chasing for the default a long time. I am not yet there because the problem occurs only when driving and randomly.

The only thing I wish I could have tried is to measure fuel pressure while driving.

Good luck