Severe voltmeter needle oscillations

Thanks Sirs, for trying to help me.

No, the negative side of the voltmeter is connected to a good ground (the metallic frame of dashboard).
And the positive is plugged onto the key switched 'ACC' connector of the fusebox.

Today...
I disconnected and reconnected the voltmeter = no change
I replaced my VR with a brand new one = no change
I sprayed some electrical lubricant onto the bulkhead connectors = no change
I tried other turn signal flasher units =no change

The problem occurs only with running engine, and not immediately. Only after a few seconds.
With only the ignition key "on", no problem.
My battery is very recent, no sulfation at all.
I suspect my alternator. I will measure the big black 'BATT' wire voltage (ALT to battery) tomorrow with my digital multimeter.

François