My 92 400E did that several years ago. It needed an engine wiring harness. They get brittle and crack on 92 models. It also needed a throttle actuator I think. It was pricy, but it hasn't coughed since.
The shifting thing is normal when the ASR light is on. You have to put your foot into the throttle so far that it stays in kickdown most of the time...
The seekng for an idle speed you described would also indicate a problem with the throttle actuator. I'm not a MB mechanic, but I'm mechanically inclined. It's time to send it to the shop, but don't expect to get off easy!