engine surges (sometimes) on start-up

Sometimes,once every 10 times or so,upon start-up the engine races fast and slow like someone is operating the gas pedel up and down. The tach races between 500 and 4000. Shut the engine off, restart and it runs fine. Can't figure it out ! What gives ? This is a 1996 pontiac grand am with a 3.1

it might be the throttle position sensor.

when was the last time you cleaned the trottle body??