RPM accelerating

Yesterday on my way to work, my car started to rev its rpm's up and down and I did not have the gas pedal pushed or anything. It did not do it constantly but it would every few minutes or so. It would start revving itself and then it would stop and then a couple minutes pass and it would start doing it again. It only did this while driving. It does not do that when it is parked or when it is sitting still. I cannot figure out what the issue might be.