solved a very similr problem on my 190E 2.6

here is what I checked:

1. Fuses
2. Vacuum lines anything obivous
3. Fuel pumps
4. CIS_E system and made sure all parts met specifications
5. Wiring harness and made all the parts were getting signals from the computer to the devices
6. Sensors
7. Added techron
8. closely inspected my fuel lines and hoses at the fuel pumps, found leaks and replaced eveyrthing
9. Checked my in tank screen
10. Replaced ovp
11. Check sparked plugs, saw that they were black. I was running rich.
12. Checked the diagnostic port for codes.

The problem didnt go away until I replaced all hoses at fuel pump. I did this after I done all of the other things. While I was at it i adjusted the lambda and the car runs great now.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)