Well something hasn't been correct for a month. That's what you need to figure out.
Start with the battery, then check the wiring.
If the problem is (was) strictly the regulator point sticking, then the voltage would follow the rpm. On the ammeter this would have shown as moderate charge at idle, and only pegged charge while driving. That's because (a) the maximum alternator output at idle is usually less than 40 amps. (b) at lower voltage a battery will take less charging current. You've seen that with a light bulb relationship in light bulbs. When voltage drops they draw less current and grow dimmer.