From working on my 91 300SL ( M104 24 V ) I found the fuel distributor plunger to be gummed up causing all sorts of really bad behavior.

The motor will sometimes go lean since there is more resistance on the plunger, if reved up, held then let back to idle, the motor was rich.

Cleaning consisted of removing the fuel distributor from the air flap housing then spraying carb cleaner in various ports, applying air to the control pressure port ( be careful of spray from another port ) and spraying cleaner directly on the plunger. At least with the car I was working on, spray and air worked just fine in cleaning up 3 year old fuel.

Removing the dist consisted of removing all lines and removing 3 screws towards the center of the dist. There are other lines around the edges, don't take these out as they hold the dist together. Supposedly some plungers will fall out so be careful during removal.

There is a how to take the distributor completely apart done by a Porsche 928 guy floating around, but this is flawed as it recommends using gasket shellac / aviation sealer on the gaskets. Neither of these are fuel proof and expect gum to leach in to the fuel system.

Have a close look at the dist cap dust cover, ( the dog dish that is removable after the rotor is unbolted ) these supposedly can carbon track causing spark leakage.