Thanks for the reply Whynot,
I've gone ahead and replaced the spark plugs, with the car reacting very well to begin. The same issue of rapid RPM drop/fluctuation at iddle returned and occurred for a while until the outside temperature dropped in October. With the the cooler and dry weather, the issue has disappeared.
Would that lead to an air filter concern, even though the air filter indicator seems to be okay?
Thanks again for the advice in advance.