To Jo Scales:

Sounds like your regulator has a bad case of indigestion. Be aware that
these things don't always fail completely, e.g., just go entirely belly-up,
there are more insidious failure modes that can occur.

Another possibility is one or more of the diodes in your alternator's
rectifier pack have failed. In this case, it'll still (some of the time,
anyway) put out some DC but the ripple voltage will drive a perfectly good
regulator nuts.

If replacing the regulator doesn't fix things, have the alternator checked
by an appropriate shop.

Dan Schaefer