did you check your fast idle thermo valve and the egr valve? Although the egr valve having carbon build up might be more consistent to where the idle will stay high all the time because it gets stuck open..... but yea like inpyral said check for air in the cooling system, fluctuating idles SUCK because there are so many things that can cause them, so knock out the cheapest/easiest things to check before you start replacing things....