These are big clues!
The needle all the way over means over 30 or 40 amps is going to the battery. A battery only draws that type of current if it is drained or the system voltage (generated by the alternator) is well above 14.2 V, more like something over 15 V.
Jumping needle could mean several things. If it was moving up and down with engine rpm, then the voltage regulation is probably on full field, full time.
If it seems random; its almost certainly a loose connection that is making the voltage in the blue feed wire see low voltage or a ground short...
Pull the VR connector off and see if it keeps on charging. The green wire brush may be grounded in the alternator. Neither brush with field wires disconnected should show continuity to ground.
