I think this problem is not uncommon on especially older cars. Usually the turn on voltage for the solenoid becomes substandard over the years. Resistance points in the circuit accumulate with time.
I have one 240d that the previous owner changed starters several times for this problem. His eventual solution was to wire a ford solenoid into the circuit so he could deliver a nice clean twelve volts to the mercedes solenoid. The ford unit will activate comforatably at lower voltages and current than the mercedes one.
Another was an old jetta I had. The trigger voltage was just too low for consistant turn on of the starter. If you have a wiring diagram and ohmeter you just might be able to locate where the resistance in that circuit is.