2000 Lincoln Navigator strange idle and shaking

My 2000 Lincoln Navigator is having a problem with the idle when I start. When I start the car up it idles at 1500 rpms and then dies down below 1000 rpm, when I come to a stop the car will shake a bit, not anything severe, but enough to hear rattling inside. At moments the idle will raise slightly and drop down over and over again. Sometimes it will make a deep pitched sound and then feel like a miss in the engine, have any ideas?