Thanks for the update.

Which battery did you end up purchasing?

Figured -- glad its fixed :tup:

Actually, my idle was intermittently doing this tonight out of the blue...

I'm probably about due for a battery too... I'm due for an oil change in another few weeks or so. I'll check the battery then.

I ended up putting a light weight battery in my car:
http://shop.advanceautoparts.com/p/a...000#fragment-3

I originally thought about putting in a lithium ion battery but it was cost prohibitive. Here is the thread for that:
http://www.the370z.com/engine-drivet...n-battery.html

Had the same RPM dip together with slight batter voltage drop when rolling to a stop at neutral (idle drops from 650ish to almost 500 RPM) (mine is 6MT). After a month's trail and error I finally fixed it. Since my cause is different than the original post's, so i though of sharing it.
The RPM drop happened about 3 out of 10 times back when my car was completely stock, but I didn't bother to fix it. Then the problem worsen after I replaced the stock intake to Stillen G3 CAI. It happens almost 90% of the time and I finally decided not to be lazy and treat her right. Thanks to Z family's posts I listed the potential causes and rearranged them in the order of least to most workload:
1) Gas: I used to getting gas from Costco. So I switched to Chevron. Nothing changed after 2 full tanks.
2) MAF sensor: cleaned it throughly and still nothing changed.
3) ECU reset: I thought the RPM drop might be caused by not resetting ECU properly during my install. But i don't want to go thought the hassle to reset power window, radio, service reminder... list goes on. To avoid all that, this is what I did. Most people have seen the crazy "cheat code" and here is one example: http://www.the370z.com/nissan-370z-o...ecu-reset.html
(post #6 by cdoxp800). Tried it but still nothing changed.
4) Throttle body: I was trying my best to avoid this since my CAI requires me to take the front bumper off (yea, once again being lazy). But with options running low I ended up cleaning the throttle body. I used this as my guide: DIY Clean your Throttle Bodies - MyG37
I was surprised how dirty mine are for the millage I have (45K miles). Got lucky and problem fixed!!! Now is day 3 since the fix and haven't had RPM dip with or w/o AC on.
5) O2 sensor or MAP sensor: luckily I got my problem fixed by #4. Otherwise I would of gone through the hassle to put the stock on and take it to the dealer hoping whatever they can find will be covered by warranty.

I've got the g37s, and I was having a similar problem, battery just changed. Try using seafoam for your fuel injectors and vacuum system, as well as cleaning out your upper intake manifold.

My car runs like new again.