Why my rps rev up & down when car is parked(idled, running for about 15 - 20 min

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My car revs up and down & the rpms needle moves up & down, like if I was putting my foot on the gas to make it rev. It happens when I'm sitting waiting and car is running (warming up) and in parked. but when I put car in park, it seems ok. No check engine light came on. Any ideas as to what might be wrong?