C3 Alternator Gauge

I bought a 73 with 80k on it a few months ago. I have been noticing that the alternator gauge "bounces" or moves a lot. When i am running at higher rpm's the jitter seems to get better but is still there very slightly. The gauge is always in the plus range but I also notice when the head lamps are on the interior dash lamps seem to flitter a little bit. Is this the alternator or the battery? It seems as though the car is running off the alternator and not the battery. Is this normal for the vette? Any ideas? Thanks