I had this exact same problem with my 560SEC. Turned out that the fuel filter had become clogged, and was causing fuel pressure to drop drastically. Changed the fuel filter, and it ran great!

Shortly after that, my fuel pumps both died, my mechanic said it was because of the strain that the clogged fuel filter had put on them. So they worked for a while after that, but they sounded really bad, and were on their way out.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles