civic Si 1989 1.6L @127k mile
When I stop at traffic lights, rpm drops to ~500 or 300 from normal ~750. In response the car jitters. No such jitter when moving (eg >1000rpm). Spark plugs replaced 14k miles ago (so pretty new). No record of air filter replacement at least during the past 30k miles, but the filter looks white. What could be the reason?

 

How old are the plugs wires, cap, and rotor? If has been awhile, this is where I would start.

 

Is this another possible vaccuum leak issue? Check it at least... My '02 Si hatch has been idling low and jittering a bit as well but not nearly as bad, but it's getting an oil change and a tune up tomorrow if there is a break in the rain.

I think mine is supposed to idle at or just shy of 1k and sometimes it drops to about 700-800 and that makes the whole car jitter just enough to be annoying...