idle

ok, now i got in my 98 eclipse rs, started it up and it reved to about 5,000 by itself, so i was like what the hell, it came down to 3,000 and stayed there for about 20 sec, the came to 2,000. So i figured it would go down if i drove it, it eventually did, and it hasn't came up since. Anyone have any ideas what caused this and what it was doing????