same problem on volvo
I know its not a benz, but I went through the same issues twice on two different volvos. The first one was the hall effect sensor in the distributor. Its signal feed the cylinder firing (timing) through a power amplifier, through the coil, but it also provided the signal for the tach. Sympton of seeing the tach drop to 0 rpm when cruising on the highway (ie no power then) confirmed it.
The other rpm sensor went out but just gave hard starts when hot.
I don't know if this experience helps, but $0.02 anyways.