an idling speed

[Pinaos]

Hi there!
I am curious about the idling speed of your car. The reason I ask this is my X ('05 4.4i) has a vibration issue. When I start my car, an idling speed is about 800-900rpm then it drops to 600-700rpm less than 5 min. Then eventually it goes down to 500rpm (the line between 0 and 1 ). When the needle goes approximately below 600rpm it starts to vibrate and it becomes very obvious at the 500 line. I am trying to do my homework about the vibration is just an idling speed issue or something else (bad crankcase breather, spark plug wire, bad coils etc) which were suggested here.
Anyway, I would appreciate any of you would tell me the idling speed of your car.

Thanks.