Experiencing voltage rise

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So, not your ordinary voltage issue thread.

I have replaced my original alternator with a 210a unit thats had an external regulator fitted to it. Now, sometimes when I'm sat with the engine idling playing some songs my brax volt meter up front shows that the voltage rises by anywhere from .1 to .7 volts when the bass drops, bringing the usual resting voltage of 14.4v up to the 15s. Sometimes you can feel the engine slightly flinch, but it only happens when the engine is idling or close to idling speed, i.e. when nearly coming to a stop.

Obviously the current draw from the amp isn't large enough to cause any drop in the voltage, but to me it seems like it must be on the threshold of what the alt can supply (which from memory is about 180a at engine idle), and the rise in the voltage is from the regulator almost overreacting to the sudden current draw. This wouldn't bother me too much, but sometimes I see the voltage creeping into 15.xv - which obviously over time won't be too good for my batteries.

Anyone "electrically minded" able to cast any thought on whats happening here? Is there any way to remedy it to try absorb some of that extra voltage?

Many thanks.