Had the same issue with my '89 190E 2.6 (same engine basically), but had a concurrent low compression problem also. A head job did the trick for compression and passing smog (along side a new catalytic converter), but it kept misfiring and stalling at times even after this job was done.

Turned out to be a loose 8mm nut on the ignition coil. Remove the cap and check for sparking in the dark.