The problem is most likely dirty voltage or current regulator points. Disconnect the Battery wire from the regulator, take the cover off, and burnish the points with a piece of index card, a business card, or some other form of thin cardboard.

DO NOT USE A FILE OR SANDPAPER!

Regulator points have a slight "wiping" action as they move, and scratches left by a file will cause them to stick. Don't tinker with spring tension; just clean the contacts. Reconnect the Bat. wire and see if things improve.
Jerry

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