Engine speeds up and down. on 2001 Ford Mustang

Engine seems to speed up and slow down even after it warms up. I read somewhere that something dealing with the throttle body needed removing and cleaning with carburator cleaner. Can anyone explain exactly what needs to be done?

Asked by for the 2001 Ford Mustang
the deposits at the throttlr plate can cause this kind of issue
more than that, the idle air valve is a more common failure. have it check for operation.