I had just similar problems, well first was easy, corroted connector in the main supply line to battery.
But after that, maybe once a week it raised the voltage over 15volts and more. I solved it by making relay to the system, so the switched 12V which is normally coming from bulkhead and supposed to go to the regulator and + field by original wiring, is now just controlling the relay, and 12V is coming directly from battery to regulator and + field.
No problems since.
