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I will almost guarantee you the problem is the harness that is between the alternator/starter/post on side of transmission.
There was a manufacturing problem with these harnesses and they will fail at the terminals.
I have changed quite a few....
With the car running and a freshly charged battery check the voltage at the alt, then the starter, then the post on the side of the Trans, if its voltage is any more than .5 of a volt difference to what is showing on the battery then that's the cure.......it is a pain to change without pulling the intake, but doable with a lift and small arms.....