Help with Volt Gauge flicker.

First, what are we working on?

Next, what do you have for an alternator / regulator, IE Mopar, Delco, etc?

"Generally" the thing you just described is caused by VOLTAGE DROP in the harness, and involves a connection / contact making poor contact. "One" scenario would be a poor contact in the ignition switch. There's some dirt / corrosion/ whatever, and the sensing/ power lead that goes to the VR "senses" low voltage because of the poor connection. The VR tells the alternator "ramp it up." The voltage rises, the connection sort of "makes" better, and suddenly it's TOO much. The voltage drops off.

This does not ALWAYS happen this way. Every once in awhile, it's caused by a "loopback" condition in the harness or ground. It can also in rare instances be the VR itself.

The fact that you jumpered to the battery, and it "fixed" the problem, indicates you are on the right track. ONE WAY to work around this is to use the key to trigger a relay, and then power ignition loads and the VR / and field off the relay. you want to fuse the relay contacts, come off the starter relay stud. You can put the relay either under the dash or under the hood.