Focus fluctuating idle speed

[Roley130]

I was wondering if anyone knew why, on our immaculate low mileage 3 year old Focus 1.6 Zetec, the idle speed 'see-saw's when the engine is warm but not hot. This usually ocurs if you start it say an hour after it has been driven. It has always done this, and it doesn't seem to effect the driveability, but it is bizzare. Is this just an idiosyncracy of these engines ?