[Roadster]

I also have the same problem on the 94, but goes away as Mark mentioned.
Still it is noticeable, but doesn't effect how the Z runs or drives, which is a good thing.
Depending on the alternator, if I remember correctly, the stator has to be "excited" in order to maintain the proper voltage, and this does not happen until the engine reaches or is revved to a certain rpm. (usually around 1500 r's)
This was also prevalent on my 82, I would just make sure I brought the r's up, although the dash lights weren't flickering, the volts weren't where they were supposed to be.
IMO, if the flickering goes away after a few seconds and doesn't happen when your driving, I wouldn't worry about it. Just make sure, even with a "cold" engine you bring the r's up to 1500 and you shouldn't have any problem.
On the other hand, if it continues when driving at different speeds, then there is something that isn't right, which could be the brink of something "big", (like getting stuck on the highway) or something real "small", such as a short, possible wire break, etc., etc, which could turn into something "big", if you never check it out!!!.......