As above, first thing I'd suspect is bad/ worn brushes, or dirty, greasy, worn slip rings.
Second thing I'd suspect is a poor connection in the charging line. That is, the path from the alternator, through the bulkhead, through the ammeter, back out the bulkhead (red) through the fuse link, to the starter relay, and the battery.
There is a big factory splice in the black ammeter wire, taped up in the under--dash harness. To find it, you have to remove the cluster, and untape the harness from the black ammeter ring terminal down to find the splice. We had a couple of cases where this splice had failed.
This also you could be right in/ at the ammeter, or breaks in the wire to the molded wire ends at the ammeter.
Your number one suspect is the bulkhead connector.