While I'm sitting idle at traffic lights I put the car into neutral. The engine stays idling at about 1000 RPM. However every now and then it drops down to about 500 RPM and then quickly back up to about 1200 then goes slowly down to 1000 and stays there for a bit until I go again or it drops again.
It only happens for a second or two and about 3 times a minute.
Is this normal or does it indicate some sort of problem with my car?