[MOTORHEAD]

All batt current flows thru the ammeter shunt (shunt can be internal to the gauge , or external ) most automotive are internal. If the shunt is intermittently opening due to high current, bad connection (internal) etc, you lose all power except batt to starter power.
Remove the ammeter and connect the ammeter feed wires together and insulate the connection well. Easy to do. At least you will eliminate that as a possible problem, and if all works ok, put in a voltmeter.