After startup my idle starts to bounce from 1000 to 500 and even below. The idle get violent when bouncing up and down and you can even feel a shake once it goes under 500. This happens about a minute after each startup and I just usually drive off. Recently while watching the idling, the car has cut off while in park. It doesn't leave or activate a check engine light. I have tested both the alternator and the battery and they are good. Where should I go from here? The car has 88k miles and I just got it.