I had very strange gauge and other electrical problems on a non-Mercedes vehicle once. To some extent I attribute the previous owner selling the vehicle because of this because the altenator light stayed on all the time.
It was a short in one of the dual-element turn signal bulbs. When one of the filaments broke, it fell down against the other filament. This created an electrical issue because power would be applied when and where not "expected" by the system.
A simple inspection of all your dual element bulbs is a cheap attempt at a fix. I think in most Mercedes only the front turn signal bulbs are dual element. Take them out and look for a broken filament that might be fallen against the other filament.
Man, I hope this is it.