Don't discount something mechanical. Maybe an occasionally sticking throttle cable, or dirty throttle body butterfly. It only takes a slightly open butterfly to affect the idle. When doing some work that required me to remove the throttle linkage setup, and when I re installed everything, the bracket that holds the cable was every so slightly not in the previous position, although it's bolted on. This had the effect of the cable slightly opening the butterfly, and therefore increasing the revs a few hundred rpms. Also if the butterfly and or mechanism is all dirty, this might prevent it from closing fully on occasion. So just something to be aware of and tick off the list.
