Weird overcharging issue

What are we working on, and what do you have for a regulator? If this is the old mechanical (69 and earlier) I'd replace it.

If it's 70 / later (isolated field) you might have the green connected brush holder grounding intermittently, or the green wire might be going to ground some place in the harness. And, certainly could be a bad regulator

However, is it actually OVER voltage or just showing a lot of charge on the ammeter? Could be that it's been "dropping out" and then suddenly cuts in and tries to make up. Put a voltmeter on it and see what it's actually doing.