Actually, I just had the same EXACT problem fixed today. My RPMs would go up to 2000 sometimes, then drop all the way down to 1000 and go every number in between. I know what I did, but I don't remember the names of everything, cuz I'm new to car stuff, so bear with me, k?
Depending on what Honda you have, I don't what you drive... well, I'll just go with how my car is and you can adjust to that.
I have an 89 Honda CRX, and the throttle body (?) is located to the left and under the fuel injector. There's a cable connected to that. If you pull on the cable (the throttle I think) your engine will rev. Just to the right of that is a screw-looking thing. Using a screwdriver, turn it either right or left to go higher or lower.
I did that to mine today and the idle doesn't jump around and it stays around a steady 1000 RPMs.
I'm sorry I didn't know the names to stuff, but I hope that helps your problem! If you want, I can explain it a little better and look up the names to things so you know where you're at, just let me know.
Good luck.