I don't drive my '93 after dark that much, even less during the summer
when it stays light later.  I spent the last week at the SCCA Runoffs
and the days I drove to the track in my Miata and stayed late enough to
drive home after dark I noticed a problem.  When stopped at a light or
stop sign with the headlights on, the idle drops, sometimes low enough
to almost stall, just barely staying running.

Is this just a matter of adjusting the idle (I can do this, I've done it
before) or is it a sign of a potential problem elsewhere?

Thanks,
Jim