Something else to look at; potential ground issues with the instrument cluster. I had issues with my gauges sporadically flicking around, including one time when the whole cluster went dead for a moment, then came back a few seconds later. I performed the repair mentioned below, and viola, no more gauge flickering, save for a bouncing speedo around the 45-55 mph marks, but as I understand it, swapping for a new speedo cable will fix that.

Mercedes 300D 240D w123 Gauge Cluster Short DIY Fix | Up-Fixin | The Motoring Journal

It's kinda hard to see what's going on in this Internet article's picture, but once you get the cluster out, you can clearly see where to attach the new ground wire, both to the cluster and the ground stud (bolt). You'll see what seems to be about 100,000 brown ground wires coming to one location (the ground bolt). That's the chassis ground point; the other can be seen in the pic above.