Drive it until it gets hot, then touch each spark plug and see if one is cold. If so you probably have a fouled plug, bad wire, or distributor cap. If no difference I would look into the points or condenser. Also if you haven't already get some non resistor plugs. I am running NGK BP5ES gapped at .034 in my 66 230SL. They made the biggest difference of any single thing I have done to my car. Maybe just take the car out and run it hard. If you do find a fouled plug I recommend not putting in all new plugs right away. You can end up, because that cylinder is soaked with gas fouling your new plug right out. Instead get your car hot and while it's hot put one of the good running hot plugs into the fouled cylinder and one new plug in the non fouled cylinder. A hot plug that has been seasoned will fire a fouled cylinder much better than a new cold plug. By hot I mean temperature and not heat range. Also if the plug is fouled turn over the car a few times with the plug out to help dry the cylinder out. I have actually started engines before with the plug out and ran it like that for about 30-45 seconds to dry out the cylinder. If you have ran the car for a while with a fouled plug you may want to change the oil because there may be a chance the fouled cylinder blew some gas past the rings and thinned the oil, long shot though. Best to be cautious with your Mercedes.
