i know this is long, but it will help you.

idle speed is controlled by 2 things: eacv and fast idle valve. eacv varies amount of air that bypasses throttle plate in response to signals from ecu. after engine starts eacv opens for a short time to increase idle speed. activation time is dependent on coolant temperature. when temp is low eacv is held open to maintane faster idle speed. when engine is warmed up it is activated only to maintane minimun idle speed.
The fast idle valve allows additional air to bypass the throttle plate into the manifold. increased idle speed is prevents engine from running erratically during warmup. fast idle valve is controlled by a thermo wax plunger which contracts when cold and expands when hot.
here are a couple things to check out--- a low coolant level will not activate the fast idle valve, which in turn will give you irradic idle. another thing that happens quite commonly on prelude and accord is that the thermo wax plunger sticks in the top of the valve and keeps allowing extra air to bypass the throttle into the manifold, so the computer sees the extra air (sees it as high idle) and it trys to adjust the idle down, then the idle gets to low and it tries to bring it back up, but with the extra air, idle is too high, so it trys to bring it back down again, and it repeats indefinately.
so check coolant level and check fast idle valve.