I have a 200 amp alt in my monte carlo, it was rewound a couple years ago by a local shop.
It holds up *extremely* well, but below ~1000 RPM, it doesn't put out much power... and this is how every big alt I've seen is... whether it be excessive/OG etc... the only ones i've seen anything at idle are the big stock ones. If your fluctuating at idle, the alt is probably on/off/on/off, or at least putting out just a little bit of power.
As long as you have a solid battery system, the alt will offset the vehicle's pull enough to keep the car goin almost indefinitely... I've never had any issues in the monte, going through traffic etc... I don't even pay attention to it anymore. I'd imagine some vehicles pull more than others at idle, and the alt can keep up with some, can't keep up with others.
Doesn't sound like a big deal, its part of having a big alt.