1995 Honda Civic Engine revving up and down.

My engine revs up and down. Its much higher than normal idle and kinda sounds like: vrooom, bah, vrooom, bah. The "bah" being the letoff sound; the cycle is repeatitive. The "bah" doesnt sputter or anything just a touch lower than regular idle. It doesnt do it all the time, isnt hot or cold specific sometimes does it after running all day, sometimes shortly after startup. Sometimes it continues for a bit sometimes it stops revving shortly after starting. You can only hear it when clutch is depressed or in neutral. It continues under load you just can hear the letoff. It does make my car seem a little jumpy as the engine is slightly revving continuously.