" ...some sort of booster for the coil ..." This makes me think that there was an added electronic ignition unit, like a CD unit. I know that Delta and PermaTune were very popular in the 70's to help keep the plugs clean on German cars. I have and still use one. Often the primary coil wiring is changed. If you can get close to the coil and see if only the factory wiring is connected. Look for any modifications to this wiring. Ideally, get a copy of the factory electrical diagram for the engine to confirm.
Additionally have you tested the coil? Is it good?
I had this problem in my '65 911. Just couldn't figure what the (&*^%&%*#$ was going on. A friend suggested changing the coil. Suddenly all the problems went away.
My 2 cents.