1980 911 SC idling problem

[artsalo]

Help! I am having an intermittent problem with rough idling, sputtering, etc. This morning, while stopped at a red light, idling at it's usual 1000 - 1100 RPMs, it suddenly dropped to about 800 -900 RPMs for no apparent reason. Pulling away from the light, it seemed to lack it's usual power. Although it ran better at higher RPMs, it continued to idle at 800-900 RPMs and it was not running as smoothly even at higher RPMS as it had before the sudden drop in RPMs at the traffic light. This same problem has occurred in various ways several times in the past few weeks but always seems to eventually clear up on it's own and the engine will run absolutely beautifully with plenty of power. Since this is an intermittent and not a continuous problem what is it's most likely cause?????