I would start off by checking the fuel pressure. I had similar problems with my 300CE. The fuel pressure regulator ended up being the culprit. I also had a small fuel leak from the EHA valve. After replacing both of these parts it felt like a completely new car. When was the fuel filter last changed? Mine looked quite old when it was changed out last year and I wouldn't doubt that it was partially obstructed from many years of service. Always start off with the simple things and then work up.

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1989 300CE