Alternator surging

mine does it
mine has done it for 20 years. i have investigated the hell out of it and have come to the conclusion its just like that....

just depends on how charged the battery is and how much load the car puts on the alternator.
if i stick the fan on or the headlights it stops
then after a while it stops anyway as the battery is up to full charge again.

mine will sometime ping between + and - for half an hour into a drive and then settle

mine had a points based regulator it now has an electronic regulator and the symptom is much the same battery charge level on a knife edge between being just enough or not enough.

more likely with a hugely over specified battery and a standard 36 - 60 amp alternator.
My battery same size and config as standard, fits the tray not too tall, but i just purchased one with cranking amps good enough to start a diesel truck.... unknown brand so went for overkill at low $$$ cost been in there 5 + years

can take quite a while to catch a huge battery up after it has been allowed to discharge to a level that is low but due to its size still allows the car to start easily

bog standard battery less capacity easier to charge up so the period of gauge wobble is less

this comes down to the way a battery charges

constant current 30+ amps with little voltage gain 12 v
constant current with voltage gain 25 amps 12.5 v
reducing current for little voltage gain 3 amps 13.9 v
constant voltage with near zero current to top it off 1 amp 14.5 V

somewhere in the 12 - 13 volt region if its still sinking current, you get gauge wiggle