<p>I have exactly the same issue with my D7100 and SB700, underexposure in a sequence of shots without changing the conditions (same subject, composition, natural light etc.). And I know I am not the only one with this issue (and don't have it with my D300 with the same flash).<br>
There are indeed hints of a not yet public firmware update that solves this issue.<br>
One from Romania<br>
http://nikonites.com/d7100/18603-d7100-ttl-external-flash-2.html#post236113<br>
And one from Australia<br>
http://www.nikonians.org/forums/dcboard.php?az=show_topic&forum=329&topic_id=31287&mesg_id=31287&page=#33424<br>
Hope a public firmware will solve this issue soon for all D7100 users.</p>