Originally Posted by JimBlake

IACV = idle air control valve
It's usually mounted to the back of the intake manifold, looks like a little motor. The ECU controls this to hold the idle steady. It can get sticky, so take it off & clean with throttlebody cleaner.

FITV = fast-idle thermo-valve
It's mounted under the throttle body. It's job is to raise the idle when it's cold. Clean it too, plus I guess the guts can get loose & move around which makes the idle unsteady.

Either one of these can be affected by air pockets in your coolant, so bleed out the air carefully.