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It sound to me as though it has a loose connection. You obviously have a voltmeter. What is the voltage at the back of the alternator at the battery B+ post with the engine reved up to about at least 1,500 rpm?
If it's over 14 volts there, replace the think wire going from the alternator to the battery.
Here is the wiring diagram, make a note, there is a fusible link in that wire, just incase it does need to be replaced the new wire will have to have that fusible link.
What was the voltage at the back of the alternator and battery?