If the harmonic balancer pulley rubber bonding let go and was that loose I think you would see it wobbling. A simple test is to connect a timing light and see if your APPARENT timing is way off or moving around. Another way is to make a line with paint or whiteout across the front of the pulley, run engine then stop and see if the inner and outer pulleys pieces stay in alignment as indicated by the line.

If the voltage is stable at higher speeds I would not worry too much, perhaps it drops when you apply a load such as the brakes or other lights. Why not double check with a real meter? Have you checked the alternator brushes? Belt tension OK?
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David Hunter