I don't remember this behavior when she was new, but I'm getting a bit of hunting for RPMs at idle. It cycles
a few times a minute, maybe +- 75 RPM. Happens with AC on or off. My ignition coils have been replaced, so
they're likely not the cause. It's not much of a bother, but if it's something that's likely to worsen, I'd rather take
care of it now. TIA.

'07 G35S 6MT 140k miles (slate blue, in case it matters)

 

Does it do this in relation to moving the steering wheel?

If not it's probably a vacuum leak but you should definitely check for hidden codes.

 

mine does this every once in a while. mostly noticeable with a/c on when it's super-hot out. doesn't bother me, as the car performs as it should. you will know when you have a vac leak, believe me.