[hotroddaddyo]

Specifically confirm proper ground of negative battery cable and any ground wire/cable/strap between engine and frame.
Perhaps your starter isn't getting sufficient ground source, so maybe try connecting a single jumper cable from an appropriate clean location on engine block to an appropriate clean location on frame (to create new/additional grounding)... if nothing yet, try jumper from engine block to negative battery terminal.
Heat expansion can compromise connections. I am thinking ground issue as you stated turn signal operation affects voltage gauge.... check that alternator wiring is secure, too.
If still not resolved, I recommend you check out the wiring to the starter solenoid.... unfortunately, it likely requires pulling plenum.