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Cleaning all the grounds is a good start, also the dash gauge is less trustworthy than a campaign speech, measure at the battery and the jump post with a real meter.
Common problem on the 928 is the old wiring and switches that can cause a drain on the battery even with the key off. This kills a new battery, then the low battery kills the alternator, etc. Testing the alternator is pretty brainless, they spin it with a motor and measure the output, so if it tests good, I suspect its still good.
Another problem can be the exciter wire to the alernator, but start with cleaning grounds and proper measurement of the voltage.
Nice Car, welcome to the group. These old beasts do shine up pretty don't they?
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