I might try going through every single connection on the engine side to make sure they are clean and tight. Remove both ends of the battery cable and use some scotch brite and an old style battery post clamp cleaner. They still sell them at most parts places. Disconnect each connection from the battery positive to the voltage regulator, and the regulator to the harness and the generator. take them off and clean them good. Make sure there is a good, in fact very good, ground. Add a ground strap if you have to, and make sure there is a ground wire at the voltage regulator. Also check the wires at the coil. Remember, star tooth washers are your friend.
Doing the above is easy low hanging fruit. Once you are confident that all connections are clean, tight, and that you have sufficient grounding, then you can move on to other things. Me, I would check the timing next.
