Lights flicker and gauge jumps around???

Does it change at all regarding rpm,
or is the variation constant?
Does it change with load on the electrical system, better or worse with headlights, or ni diff?
At idle it is the same both under a load and not. Cruising down the road it does it under a load (headlights on or turn signal on) but when not under a load the gauge sits steady at 14v.