When I leave for work in the morning I move the car out of the gate then get out to shut the gate, leaving the engine running.

Sometimes the engine will drop revs enough for the ECU or whatever to suddenly increase revs to stop it stalling.

It will keep doing this:

BBBBBRRRRrrrruuuummmm....
ECU kicks in....
BRUM! ......

BBBBBRRRRrrrruuuummmm....
ECU kicks in....
BRUM! ......

Is this normal on a 2.2 DTi?

Idle control bit sticky?

Most of the time it's completely fine. Just seems to happen some mornings, wet or dry.