Use a multimeter and test
distributor cap
distributor rotor
all ignition cables and their plugs
easy to do. Details are in the Bentley manual. If you do not have a Bentley, just test them all and the one which is different is faulty.
I recently had a friends E32 750 for checking here, engine was runniung really rough. Found the distributor had haircracks, 8 out of 12 ignition wires did not have the right Ohm.