Well, the questions moderately vague.
If the idle just shoots up for a second, thats no problem, when the motor is being dragged down by the transmission, as soon as you put in the clutch, the motor will be free to race.
Think of it as if your pushing against a stone wall, the wall doesnt move at all, and then bam! A crane pulls the wall over. All your weight is into the wall, and when the wall gives out, you fall over towards it. The motor is trying to race, but the weight of the car wont let it. When you relieve that pressure, the motor "stumbles" up in RPM, then catches itself and comes back down.
Now if thats not your question, and your question is the idle races up then down, then up then down, until you come to a stop. Well. My best vote would be on the "IAC"(idle air control valve)
When you let off the gas 100%, the motor gets no air, thats what the IAC fixes. The IAC opens slightly to let air into the motor, just enough so that it doesnt stall. When your mashing that gas, the IAC should close, its not needed, but when you let off the gas, it opens up. When your moving, the air is being mashed into the intake, if your IAC has a bad seal, extra air can get in. And if your idle bounces, then maybe the IAC isnt completely dead, and the computer is saying "You nincompoop! They let go of the gas, give the motor some air!" And the air is rushing in, races the motor and The computer says, "Whoa wait, the idles coming up, but my TPS (Throttle positioning Sensor) isnt saying anything!"
The computer can then go into "Open Loop mode" and try and stop listening to the sensors. And well. That aint good. So the idle will go crazy until *viola* You stop, and the IAC goes "ok, ok, ok, I got it" and everything works back up again.
So, in a summary, try testing your TPS/IAC/Oxygen sensor.
Hope this helped!
Friday, September 19th, 2008 AT 9:17 AM