Unlike domestic radio suppresion ignition cables, the Germans use real copper wire for their plug wires and have ceramic resister ends (at the spark plug and dist cap) that does the RF suppresion. These open circuit quite often and are the single most common cause of single cylinder misfires on these engines. The open circuit voltage is obvious on a secondary ignition scope. The problem can even be seen when the symptoms aren't evident.
There are many other causes of single cylinder misefires but identifying the cylinder is usually the first step.
Bosch Master, ASE Master, L1
33 years MB technician