First Mercedes -

Your comment that it ran well after a refueling might lead me to believe that the fuel screen in the tank is partially plugged with dirt. When you filled the tank, the incoming fuel displaced part of it and allowed it to get sufficient fuel through to run OK. After a while, the dirt would accumulate again and restrict the flow.

I know that you said that when the car is sluggish the engine still revs OK in neutral. It doesn't take a lot fuel to rev the engine that way.

Just my thought on the problem. It will be interesting to see what you eventually find is the culprit. Keep us advised.

Thanks,
Wes Bender