Without knowing what you tried, that can often be attributed to dirty fuel injectors or bad spark plugs. Check the spark plugs and see if they are dirty, and if they are just replace them and rule out the possibility. As well as that, you may want to run something like Techron through the fuel system and see if that helps clear out the injectors. You can do both at once, or one at a time. Same with the air filter; check for dirt, and replace the filter if it's dirty.
Those Benzes also tend to have a wire throttle body (sometimes called a DBW throttle), so check the wire and throttle body aren't damaged. You can also hold the wire down by hand and you do this with the engine idling, not hold it and then start. If you hold it down, don't do it too long - 15 seconds max. You want to see it sustain a load, not test the entire RPM range.
The other option is to try an Italian tune-up as well, if you can legally do 60-80mph and keep the engine RPM high (and can be safe while doing so). Sometimes if just Techron fails, you need to combine it with an Italian tune-up. Where I'm from, fast driving is mostly normal, so someone doing an Italian tune-up isn't be unusual but YMWV on how "normalized" speeding is. It's also kind of gas intensive, so erm keep that in mind a bit. These older i6 engines don't drink as much gas as a S600 with the M120, but it's a definitely a factor during the gas crisis.
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