6V alternator weirdness.

[Simonpie]

This is on a 6V positive ground with an alternator. It has no external voltage regulator. I ASSUME there is a simple solid state one built in. I bought the car like this, so I don't know the source or history of the electrical stuff.

So here's the story. The volt gauge may be a bit off in calibration, so don't harp on the specific number too hard:

When sitting, engine off, battery charged and nothing on, the voltage is 6.5.
When sitting, engine off, lights on, the voltage is 6.3.
When sitting, engine on, light on or off, the voltage is 6.8.

So far so good.

Sometimes, driving down the road with my lights on, I notice the voltage at 5.5. I freak a bit, which doesn't fix it. I tap the gauge. Again, no cure. Why do I like tapping the gauges on old cars? it just seems like something I should be doing. I pull over and start calculating whether or not I can make it to my destination and back without a charging system. I turn off my lights and instantly the voltage jumps to 6.8. I turn on my lights and it stays at 6.8. Problem solved.

So the question is: Do I have a sticky voltage regulator? How long can I use the flick the lights trick before it stops working and I get stuck on the side of the road?