I had a similar problem on my 88 300e some months ago. I replaced the vacuum line that runs from the air filter down to the fuel regulator (I think) and that solved the problem. My car had got to the point where it was even stalling now and then. The vacuum lines may appear to be fine, but you need to check them with a guage. I'll bet you have a vacuum problem.

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Oupa
1988 300e