Had the same thing on my 88 560sel - did all your stuff same thing. What worked for me was replacing the fuel lines. They were so rusted inside that one broke in two from it's own weight when we removed it. The rust is not trapped in the fuel filter as the filter is in the back of the car and the fuel picks up the rust and takes it to the fuel distributor. Take the injector lines off the fuel distributor and carefully pull out the screen in each injector line hole. I think you will find them partially clogged with dirt. I cleaned my screens and replaced them - you can put new ones in - but check your fuel lines first. Before i changed my fuel lines - I cleaned the screens and the car idled perfectly for about 100 miles and started to act up again. Try cleaning or replacing your screens - they are cheap from Phil and if the problem goes away - then you know to change your fuel lines. The oil guage on mine fluctuated the same way - the tach barely showed the missing but you sure could hear it and feel it - it was annoying. Hope this helps you.
Just had a crazy thought - if you do have rust in your lines and want a cheap fix (I didn't think of this until now!) cut the fuel line in two just before it goes into the fuel distributor and cut off about 3" of line and put in an aftermarket - cheap inline fuel filter with rubber hoses. Clean your screens and that should work . If you get one of the see through fuel filters you should be able to see the rust in it after you drive for a while. Surprised Mercedes didn't use one up front for contaminants from the long fuel lines.