Mine would go up and down in idle too. There is a sensor(for everything it seems) that adjusts the idle to make it run smooth. But my car is leakin oil in the motor(bad head gasket I think) and at higher rpm it runs fine. So at idle it goes up and down to compinsate for the missfire that happens due to the oil foulin the plug. But this is just an idea, I have not takin it apart to make sure yet.