I had the exact same behaviour on my 1989 Honda Accord LXi - rev up and down when warm. Very annoying. But the fix was easy:
What you most likely have is a dirty IACV a.k.a. Idle Air Control
Valve that causes this. The sensors there can get dirty sending erratic signals. So I cleaned it without even uninstalling the thing, just loosening the two bolts and using a straw from the spray cleaner to get inside and in between the crack to clean the sensors and and viola, the engine runs like new.
Just be prepared for the rubber gasket to fall out. You'll have to fish it out and place it back as best you can before tightening the two bolts again.
Here's more on this issue:
How To: Clean Your IACV - Team Integra