Well something isn't right. Can't have 13 volts at the Alternator stud and battery but just 1.4 volts at the bulkhead connection.
If its real, something is loose and making intermitant contact. Or worse, there is a chafed wire and its grounding. But if was shorting on the main splice side of the ammeter - you'ld see the ammeter bounce to higher discharge whenever it contacted.
