Fuel-injected? Carbureted? Engine warm? Engine cold? Did you change gas
suppliers recently? When was the fuel filter last changed?  When was the air
filter last changed?  Is the engine otherwise producing good power?   Any
blinking lights on the dashboard?

There are many items which could lead to unstable idle speed.  EGR.  PCV.  Fast
idle valve.
--
Chuck

In my experience, the problem you described is generally caused by
either a faulty IAC valce (fast idle valve) or poorly beld coolant
lines. To troubleshoot, I'd start by bleeding your coolant lines as
recommended by Honda. The bleeder screw for the coolant on your Prelude
is located on the thermostat housing just to the right of where the
upper radiator hose goes into the engine.

If the problem continues after you've properly bled the coolant lines
then I would suspect that your IAC valve has worn out. Sometimes they
can be repaired, but often you'll need to purchase a replacement.

Hope this helps.

-Spiff
http://www.preludedriver.com