Is it an original DC generator/regulator system? The mechanical regulators have a habit of developing dirty points, and can cause the ammeter fluctuations you're describing. The points can be cleaned, but resist the urge to use a file, or even sandpaper on them! A piece of white business card, or a file card is just the right roughness to polish the points. Disconnect the battery wire from the regulator first, because accidentally pushing the cutout relay points closed with the engine stopped will cause them to stick and burn.
Jerry

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