I had the same apparent problem with my '89 300E a couple of years ago. It idled rough and occasionally stalled while driving and releasing accelerator. My dealer had no problem diagnosing the problem as UV Protection Relay and O2 sensor. He said the UV Relay was the major culprit. I elected to replace it and the stalling stopped. It still idled rough so I then replaced the O2 sensor and now it idles smoothly. Total cost was about $150.00. The relay is very easy to do and a nimble fingered weekend mechanic should have no problem doing the O2 sensor. The wires for that go thru the transmission tunnel and connect under the passenger side carpet. If you now the O2 sensor has been replaced in the last 60K miles, go for the relay.