So that's one of these few m104 with CIS? That might be the potentiometer of the air flow meter. It tells the electronics the amount of air entering the enginge so it knows how much fuel to inject. Typically the potentiometer wears out first in the range above idle. It is not very difficult to replace and costs around 45$ from europartsdirect.com (not available from dealership or fastlane).
Ignition or fuel problems would be even be more evident at higher revs, and idling is fine too you say.
Good luck, Bruno

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1992 300TE 160 kmiles