Ford KA - Rev Problem

I have seen a similar post to this below my KA has the exact same symptoms. Basically when the clutch is depressed the engine over revs, it is much worse going up through the gears and when coming to a stop, the stationary
idle is a little high and like the other post I feel sometimes the engine is constantly trying to go faster. I have had it in the garage and they replaced the Idle speed controller, and for about 3 days everything was fine, then it started over revving between gears again. Are there any other things that could cause this type of thing? I was thinking and air-leak somewhere? or can it get gunked up in 3 days somehow? Any help greatly appreciated

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