Peter, how many miles on your 1992 300SL? I would suspect a problem with the ignition wire/connector for Cylinder 5. What brand wires did you buy? Beru or Bosch are fine choices. Hopefully you installed a Bosch distributor cap. Recommend installing a new Bosch distributor rotor also.

A partially plugged catalytic converter may be causing the lack of power. I recommend asking other MB owners in your area if they are aware of any reputable independent shops that work on MB vehicles.

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Fred Hoelzle