'66 Valiant Weirdness

CURRENT through a circuit causes voltage drop. What might be happening, is that the new ignition draws more or less current at different RPM. As far as the flickering, a bad connection can "break" and drop connection so to say, then current starts to drop and connection sort of arcs across and remakes. There are other reasons, but there can be what are sometimes called "loops" or "ground loops." These can cause certain interesting cycling.