I have an '89 260E and was experiencing the same problem at idle.
What finally solved the problem was switching to 89 octane gas. After the first or second tankful, there was no more shuddering/hesitation. In fact, I often can't tell if the engine is running.
This has appeared a couple of times in this forum, each of us noting the change in idle behavior after doing it. I think this will solve your problem too.