I'd suspect the voltage regulator, but first try the following:

1) Inspect the battery cables and body ground cables closely
2) Clean and reinstall the battery connections
3) Remove the ground strap connection on the passenger side fender and clean the connection area
4) Clean the body ground connection on the driver side fender
5) Remove and clean the block ground connection on both sides of the engine
6) Spray contact cleaner on the harness plug and alternator connectors

Ckeck the voltage drop between the battery (+) and alternator connector (B-02).

If it still acts up, I'd try disconnecting the light control relay (connection 3-R) and see what happens.

If it still insists on acting up, I'd replace the regulator (or alternator).

Frank