Alternator, voltage regulator, or battery?

When I’m cruising, my dash lights are just fine, my amp gauge is in the “charge” area, and everything is great, but when I come to idle at a stoplight, I drop into the “discharge” area and my dash lights get very dim. At a stoplight when I was completely stopped I dropped it into neutral (auto trans) and it kicked it up towards the “charge” a bit more, gave me a bit more juice for the dash lamps, but still in “discharge”.

My question; should I switch the alternator to a higher amp alt rather than the stock one that is in there? Is it the voltage regulator? OR is my battery not giving me enough juice?

I’m sure if I switch all three out I’d solve the problem, but who wants to do that??

The alternator is fairly new (bout a year), the battery roughly the same (however I don’t know its ratings) and I’ve never touched the voltage regulator (it looks absolutely brand new however).

The car isn’t doing anything but not giving enough power at idle.. once I step on it, everything’s great.