Patterned drop in RPMs when idling Options

[rjames]

My car idles steady when cold, but after warming up a bit the idle will drop every other second from 900 to ~600. *Sometimes* when the engine is warmed up this doesn't happen. Additionly the volt meeter also drops in unison sometimes (or at least it did today). I haven't measured voltage at the battery while this is happening yet since I only noticed the voltage drop this morning on the way in to work.

It's been doing this since I've owned it for the last 6+ years. If it was a vacuum leak I would expect the RPMs to go up, not down, right? I've done a lot of work to this car, including:

replacing all hoses (vacuum and fuel)
replaced fuel pump,
set fuel pressure,
new battery,
replaced the relay board
new voltage regulator.
new plugs, wires, set timing and replaced the points.
new injector
new injector o-rings.
plus other things I'm not remembering at the moment.

All of the above work was not done to fix the idle issue, but to solve other issues or as routine maintenance. None of them had any effect on the idle. Car runs good and strong, otherwise.

Any ideas of what I should check to remedy this?