My car is idling like sh**. When I first turn the car on, it feels ok, but after about 30 seconds, it starts to rumble. I dont really know how to describe it, but it sounds like the engine is struggling. I let the car sit in my driveway in "park" for about 2 minutes and it just started to sputter and the RPM's dropped to about 500 and floated around there. It sounded like it was going to die so I just turned it off. When I was driving home from school it felt fine, nothing unusual. What could be causing this?