You described the problem in two words- - - -"electronic ignition". The only company I'm aware of that offers a 6V model is Pertronix, and their products are notorious for gremlins like you're describing. Trash the ignition system and go back to a conventional set of points and an OEM style Delco-Remy coil. Change points every 10K miles and enjoy your ride!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!