IF the current is really fluctuating then so should the voltage. I think I would put a voltmeter on the battery. I would not use a digital but an analog so you can see fluctuations if any.

IF no fluctuations my next guess would be a loose conductor somewhere in circuit. OR a bad crimp - check for heated connections.

Could it be a load that varies ie windshield wipers (ah nuts I bet you have vacuum)

Last - is it just the nature of the instrument ie inadequate dampening of meter movement? I wonder what it would do if connected to teh truck but sitting on a separate bench. BUT I sure dont want to go to you and tear half a dash apart to answer my curiosity.

Uh this is a generator setup and not alternator. IF alternator could it be a shot rectifier?
Dan Bentler