It is a known fact that the 140 body style had this issue, and the fix was to replace the cluster under warranty. I dont know if this issue extended to your 190 car. The problem is the voltage regulator circuitry in the cluster. there have been many sucess stories in changing the capacitor for a larger value.
Before I became aware of these issues my MB service manager suggested it might be a weak alternator or regulator.
My symptoms are, using washers, wipers or directional, the seat belt light comes on followed by the rest of the indicator lamps and all the guages go to zero for a few moments and then return to normal.
In your car changing the regulator and or brushes is pretty easy, I would check voltages first, and or check for bad grounds.
http://mbcluster.com/