I noticed that all the lights, interior and exterior, were varying in intensity. Shortly thereafter, the battery warning light began flashing on and off intermittently.
A check on a charging system analyzer showed a wildly fluctuating current. The alternator was replaced. Two weeks later (I didn't drive at night in the intervening time) the original symptom 're-appeared', but was intermittent.
A check of the battery revealed a bad cell and low CCA. It was replaced.
The intermittent problem continued. On the advice of a friend with a solid automotive electrical background and an "Oh, yeah!" from the shop's Electrical Tech, we replaced the regulator gang plug on the alternator.
Sorted.
Moral: If one of the connectors in a gang plug makes a solid connection, the plug will feel solid, BUT any of the other connectors may not be properly connected. Check them all individually.

Thanks N J for sharing this ....

You're welcome. I came in from the (Classic) Mini world where the message board is a very lively and informative entity, chock-a-block with sharing.