they all do that from time to time
its a combination of battery on the edge of being fully charged or to put it another way voltage seen by the regulator from battery, regardless of its state of charge is bang on the switching point between charge and do not charge.
charge current comes through ammeter on standard car so you see it twitch with every on off on
The regulator is switching alternator off and on becasue it can't make up its mind what to do.
Temperature has an impact on how well a battery takes charge and to the regulator which in a modern one uses a fine balance of volatge across a small circuit to switch a power transistor off and on to provide 0-5 v into the field
it can't be half off or half on so it just flip flops
some times on the on off switch point can change with ambient temperature.
take it for a drive, get battery well charged and it will stop
problem can be made worse by the regulator not being able to see battery volatge or by being badly grounded which means it can't work out what the battery voltage is, by compariosn with ground ov
i.e loose on its mountings or they have corroded
in both cases check wires and mounting, bad connection at battery or between battery wire and the clamp can cause problems.
Dave