Idel fluctuates once the car is warmed up and stopped

This is a very annoying problem with my car that I can not figure out. Someone mentioned ECU problems and others say Oxygen sensor and others say power steering. I am out of ideas. The idel rises then falls repeatedly once the car is warmed up and stopped at a light. Its like the ECU is telling the idel is to high and for the sensor to drop but then once idel is dropped it drops to low and ECU tell to raise but raises to high and so on and so forth, is it just that idel is set to high or low? but this does not occur when the car is first started only once it has reached operating temperature.