The M54 engines which I would assume is in your car have issues at higher mileages with the idle air control valve sticking due to carbon build up. Get a can of throttle plate/body cleaner and remove the IAC valve and Throttle body and clean all the build up off of the inside of the valve. If your air boots going to the intake haven't been replaced yet they tend to rip when they are older. It shouldn't take more then 2 hours to do.