Check your idle air control valve, it might be malfunctioning.
zerothread?id=251133Great info on how to take off the idle air control valve, it's a bit tricky if you got big hands though.
It's sort of hard to diagnose without knowing much about your car.How are the injectors? Spark plugs? etc...
Modified by Razi at 2:27 PM 5/11/2009