I am experiencing a momentary drop in RPM when at idle.

The idle RPM will suddenly drop toward zero for a split second and then return immediately to normal idle speed. I can feel a sudden interruption in the normally smooth running of the engine (and a commensurate drop of the tach needle), followed by an immediate return to a normal idle in less than a second (tach needle never makes it to zero).

Sometimes it will occur only once, sometimes it will happen several times, with each incidence of a single, split-second idle interruption separated by a few seconds (5-15 secs between).

It mostly happens after a cold start but occasionally occurs with a warm engine when at a traffic light or stopped in traffic. It does not always occur, however. Some days I don't experience the phenomenon at all.

There do not seem to be any issues when driving with the engine under a load.

It almost feels like an instantaneous, momentary loss of electrical continuity, for just an instant. Also, when it happened once at night, the headlights dimmed dramatically at the moment the idle dropped.

Any ideas?