Rob,
As said previously, altering the idle speed may just be masking your root cause. Quite often a fluctuating idle is caused by a tiny ring of carbon around the throttle body next to the butterfly plate. Try removing the TB and gently opening the butterfly; hold it open with a wooden peg then get in there with some carb cleaner and an old toothbrush.
The reason the carbon causes the fluctuation, is when at idle, the will move the butterfly to its 'learned' position, which when learned would let a certain amount of air through. With carbon build up, the butterfly effectively closes on to this carbon and chokes the engine, the spots the engine dying and rapidly opens the throttle, only for the cycle to repeat.