Voltage gauge drop

I have an 06 Yukon. While driving several miles I notice my voltage Gauge dropping below 12. If I do not keep my RPMs up while coasting to a stop the engine stalls, but restarts with no problem. I've changed the battery.

Have the alternator tested by a shop.

The voltage regulator/brush assembly, located inside the alternator, likely needs to be replaced. Easy job for a DIYer.