when im in park and just sitting in my car my car rpm will drop down to like .5 then it will shoot back up. like every 20 seconds it does this.

is the a/c on or is defrost selected on the control??

no nothing is on just radio thats it and lights.

i dont know much about lincoln ls engines but in my other car i had that problem and it was my throttle position sensor

do you remember how much that cost you?