1998 Ford KA - Why is it reving when i dip clutch!?

Had my Ka for 3 years now, about a year ago it began playing up when changing gears this then went away and now it has come back with avengance! Over the last 2 weeks especially most times i change up or down gear the engine makes a very loud revig sound. And when i come off a motorway or even stop at traffic lights and dip clutch and break to stop it goes crazy and even if i take it out of gear straight away to slow down it still revs like crazy, sounds like its going to blow up. I have read some forums suggesting its the 'idol engine valve' but ka owners were saying their ka cut out and stall which mine isnt. Took it to garage this morning after Ka had been fine for a few days and mechanic drove it and it came back briefly i think he mentioned something about the valve. Any other suggestions??