1994 Honda Civic engine revs up and down by itself

1994 Honda Civic, mileage: 58,500. My Honda Civic's engine revs while the car is either park or stopped (with the transmission in drive). I have an automatic transmission, but this problem has happened to me three times over the past month (both when the engine is fairly cold, or when it is hot). The first time, I was in park with the engine running, and the engine would rev inexplicable from 1000 to 2200 to 1000 rpms every second. Then, when I put the car in drive and started moving, the problem would stop, and I didn't have the problem again while driving. This last time, the same thing happened, only I was stopped at a light with my foot on the brake and the car in drive. Any idea what could be causing this?