Dodgy starter motor?

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Starter to alternator likely not - that's downstream of the battery to starter connection. The wandering voltage sounds more to me like bad connections at the battery cable to the harness - which is at the back of the starter.

Dumb question, but have you had all the cables off and given them a good clean-up along with the grounds? Also, jumper out the battery to starter with a set of jump leads and see if this changes/improves things. That fluctuating reading comes down to crappy connectivity somewhere I suspect - inspect the battery connections and everything upstream.
 
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