Why does the car REV by itself?

The engine light of the car has been on for about a month. When I first turn on the car and begin to drive, the car feels like it pumps when it moves at slow speeds. Once cruising, there are no problems. Then, when the car is at a stop, the RPM drops to below one and sounds like the car is stalling and is about to turn off. But before that threshold of turning off, the car itself pumps and REVS the engine to avoid turning off. It is as if the gas pedal is being pressed in order to increase the RPM at a stop. So if I let go of the brakes, the car will throttle but not like a****** move. When the car is in neutral, this effect is diminished but still to a very little extent still pumps and REVS on its own. Whats wrong?