Another possibility: Disconnecting a battery while the alternator is running is a known cause for taking out regulators. Some systems were more prone to this than others (the 60's-70's Ford mechanical regulators would go out almost every time), but at the moment the battery is disconnected, the voltage shoots way high (like 18-20 volts or more) and that will do it. Could have been an intermittent connection like above, or an intermittent inside the battery. This would explain the surging ammeter too; the voltage surges would cause that.
