I was trying to get you to use the meter that you checked across the battery with to check the voltage at the point your inside gauge is connected to. As others have said, an analog gauge may react differently than a digital gauge but it would be nice to know.

I would guess your MSD switching is causing the flick, but, nothing like proving it.

And, unless your regulator is bad, I would still suspect your blue wire is reading a lower voltage than you are seeing at the battery with the meter.