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Idle erratic

I have a 2001 nissan Altima. when the car is in neutral, or in drive but at a stop, the idle becomes erratic, going up and down. The car does not stall but I am worried it could. Car runs fine when I use the gas pedal, no skipping at all. Is there a sensor that could cause this problem

More likely an idle air controller (IAC) problem or a vacuum leak.