when i sit in my car at idle, and step on the brakes or pull down my windows, my rpms drop to about 400 and then when i let off the brake or stop pulling up my windows it goes back to its normal idle. is this a bad alternator?? plus when i turn on the car it shoots to about 1200rpm and then all the way down to about 200rpm than to its normal idle at 700. could the be a possible vaccum leak??

 

test the chargin system on the car...see if it is up to par. and while you are at it, check the hoses to be sure