Why does my car rev on its own?

it revs up and down when i start the engine and it sounds like im pressing the pedal but im not. i hope this makes sense and someone can help. i have a vw golf mk4 1.4 16v 2001 it has a sports exhaust on it. http://www.youtube.com/watch?v=7H6w6tY1_rA i have a video here.