I have an AutoMeter voltmeter hooked up to a '78 350 SBC with a internally regulated alternator. For years, it has always read a fairly steady 14v. However, on a recent long (700 mile) drive it started acting funny. I'm trying to sort it out. About halfway through the trip, the voltmeter started jumping between 12 and 14v every few seconds, but still mostly stayed at 14. I figured it just meant that the regulator was regulating. But then it started jumping from 14 to 10 to 12 to 14 to 12 to 14 randomly. Eventually it would drop to 8v or below for a few minutes and suddenly pop back to 14 and all up and down the scale. Sometimes it would pop back up if I goosed the throttle, but not consistently. It would also pop back if I turned on the headlights or wipers, but eventually it would start jumping around again. I couldn't determine if this was an alternator/regulator problem or a gauge problem, or both. I checked the connections and they were all tight. I did notice that the fan belt seemed a bit tighter than it probably should be while the engine was hot. How can I rule out/rule in a problem? Thoughts?