Ford KA Erratic idleing

Hi,I have a 1998 Ford KA 1.3 that has delveloped a idleing problem, when I pull up at traffic lights ect the engine is raceing for a few moments
then it will settle down and idle ok, I set off again pull up and it does it again. The strange thing is I can rev the engine when it is stationary and when I take my foot off the throttle pedal it will idle ok, If I put the car in gear and drive it a few yards up the road and come to a stop the engine starts raceing again. Has anyone got any ideas what might be the problem please?
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