HEI & external regulators?

[Zonaman]

My alternator guage tends to jump around the middle mark (at idle), particulalry with the lights or signals on. You can see a slight dimming in the lights corresponding with the guage jump. Here's what I've looked at so far:

*Checked wires & grounds for good connections- they were good.
*Checked battery. It's clean, in good condition, and reads a 12v, engine off.
*Checked battery, engine at idle, lights on and off. 15v either way.
*New belt, correct tension.

The battery stays charged. The guage only does this at idle. I have heard that externally regualted alternators, which are designed to work with a points ignition set up, may do this when the system is converted to HEI as mine is. What say you? Should I be looking at an internally regulated alternator?

Thank you.