Severe voltmeter needle oscillations

There are several things that might cause this, but the very first thing I'd do is round up a second meter, even a digital multimeter, and hook it right where the one with the problem is getting power, and see if the problem "is there."

Again...we need to know about your system. Year model of car, and what exactly you using for an alternator and regulator, and ESPECIALLY "is it" an electromechanical (relay) regulator?