ok so this starts to happen after i drive for a while ...when i stop at a red light the car starts droping rmp from 1000 down to 500 then back up to 1000 it does this 3 times after every time i stop ...im not in gear at the time ......can u guys help ...is it the pulley 

it feels like the car is going to shut off on me but pulls its self together
thanks for the help
it feels like the car is going to shut off on me but pulls its self together
thanks for the help