Over revving when clutch is disengaged

Hi, I have a ford fiesta 1.3 finesse (S reg) 1998. Over revving issue occurs mainly whilst driving and when the clutch pedal is operated the engine races . I have tried the idle air control valve, the partentiometer , a diagnostic check was carried out and passed and the air piped have been taken off and cleaned and checked for damage. Any further ideas will ne much appreciated . Many thanks .