The more I think about it the more I think it is a vacuum leak ... Engine revs up, ECU sees the signal from TPS saying no throttle and cuts fuel as if decelerating, rpms go down, and the cycle keeps going and going.
One more observation:
this up and down thing does not happen when engine is fully hot. When engine is hot, idle seems to be very smooth even with electrical accessories on.
I dont't know if the original poster's idle smoothes out when the engine is hot though.
iougs-
