Reading this thread jogged my memory of this great post by mercmad6.3:
"He might not have,but I have had plenty of experience dealing with older Mercedes wrongly fitted with carbon string leads and resistor plugs. Mercedes Do not need suppressor leads as from 1937 on wards you could buy suppressed or non suppressed distributor and plug terminals for this very purpose. From the mid 1950's nearly all gas Mercedes came equipped with the suppressor plug terminals (5kohms) . From the 1970's all came with a resistor in the rotor (always check old rotors )
and then the distributor terminals as standard. Right up until the 1990's all Mercedes still had solid core leads,and for a very good reason. it is reliable and it is the best. The resistor plug terminals slow the spark down from a bright instant flash to a slow(relatively) arc almost. This Promotes better combustion . Use a carbon string lead ,which are nearly always offering too much resistance and you are asking for unreliability ,mostly hard starts and all that it leads too."
Here's then whole thread:
http://www.peachparts.com/shopforum/vintage-mercedes/349465-pertronix-radio-interference-mystery-solved.html