Try "rpm bounce". That brought up a couple threads. It's nothing to worry about unless your car stalls or revs really high (over 2000). Many people have had this happen including me and nothing came of it. There's a correllation between the rpms bouncing and hard braking, ie. "I stopped at a light and noticed the idle bouncing up and down." Usually a tap on the throttle will settle the engine down. No one has ever conclusively explained why the bouncing idle happens as far as I know, and very few have ever had it be a real problem for them.