An issue with fluctuating lights - all lights

[OregonNed]

I haven't driven my truck a lot since I bought it last December - but last night I noticed an issue. Driving home from two cruise-ins yesterday I visibly noticed every light in the truck intermittently dimming and brightening. When the lights appear "normal", the gauge seem to be sitting between 12-13 volts - but in the "bright" phase, the gauge jumps to 16-18 volts.

I'm not sure how long this has been happening, as I seldom drive after dark with the lights on. It happens as much sitting at idle as it does when I am driving. I think the connections are all good, and I wonder if it can be a connection issue by its behavior. It would seem if that was the case, the voltage would drop instead of raise - but I am no expert on it.

I have an alternator and no voltage regulator that is external, so it much be internal to it. There are no problems starting, after sitting for a few weeks at a time - battery seems charged and charging.

Has anyone seen this, or can offer some advice? Maybe a bad alternator or battery? Thanks!