IACV is the idle air control valve. It is a valve that bypasses the closed throttle plate on idle and lets through a very small amount of air into the intake for engine idling.
Many implementations of the concept exist.
My car has a (somewhat) magnetically controlled IACV. The rotating valve has to be free as bird inside its cage to rotate freely. Any sticking will cause unstable idle.
The IACV is attached to the throttle body. Like the throttle position sensor it has a few wires attached to it.
To clean out IACV, get some carb cleaner spray (STP type of thing is good), remove the plastic air intake pipe attached with throttle body, there is usually a small hole inside the TB just before the closed throttle plate, spray small amount of carb cleaner in there.
Do it while engine is started and keep the engine from dying.
Its better to take the TB off and clean out the IACV. Sometimes the IACV has screws etc for adjustment. Do not touch them (unless your mech has already).
So your problems could be IACV, or extra-tight/un-even tightness of tappets.