Intermittent Slight Stumble at Idle
 

Battery is fine.

No intake leaks (replaced a damaged breather, which I thought was the cause, and tightened all hose clamps).

May or may not be more prone to do when A/C running.

Air filters have less than 10K on them.

Any ideas?

I'm down to trying to clean the MAF sensors (I have proper spay cleaner) or maybe having my accessory belt checked for some slip (but under 35K on car...), but both seem unlikely to me...

Nobody?

Well, I re-tightened all clamps when engine was cold, cleaned some debris out of the airboxes, and seems happier. Belt is fine and tension is good.

Guess if it acts up again I'll try cleaning the MAFS.

If anyone has another suggestion, let me know.

My hesitation problems all went away after my UpRev tune.

It's not a hesitation under acceleration.

At idle, in gear it will suddenly drop to about 500 RPM and then go back to 700ish.

That sounds like vacuum. What had you done prior to this starting?

My car has done this before, never did figure out what it is.
Though I thought mine might be caused from my SC and tuning, usually mine occurs if I am idle for longer than 60 seconds.

cars nowadays are tuned from the factory to idle at just above stalling in an attempt to save fuel. a little studder in RPMs isnt anything to worry about. IMO

I wouldn't worry about it. Mine occasionally does it, particularly on really warm summer days. It's never stalled, just a random stumble. 99.999% of the time she runs like a top, so I just ignore it.

I wouldn't worry about it. The S2000 and the beast do it too. It's most prominent on a warmer day with the A/C on.

If yours is a manual gearbox, maybe you've got air in the system. Bleed the slave cylinder for air.