How do you test your IAC- I need a little help

I have problems with my 99 mustang gt convertable being stopped at a stop light with my foot on the brake and the car having a major rpm surge and wanting to take off and break out of the stop. It has almost gotten me into an accident a couple of times until i recently put hawk brake pads and drilled and slotted rotors from summitt now atleast the brakes hold it back but its still having rpm surges while stopped. It drives fine, it is an automatic and sometimes i also feel a really bad buzzing vibration in the gas pedal when i hold it at speeds around 30mph but i dont know if that is related. Any ideas? Thanks

Yes they do surge in park just not as much, when I put it in gear they drop to like 200 then it recovers and bounces between 5 and 700 until I hit the road. Any ideas about the gas pedal vibration?

I looked it up and seriously can't find any info on just the gas pedal vibrating every now and then at certain speeds. Its not the motor mounts or the driveshaft, I wonder if it could be the throttle body and be coming from where the cable connects the pedal?