I had the same issue a few times and it appears to be when ive been braking up to a junction or in traffic with the clutch dipped with ma focus... Its a 1800... Mine has cleared up on its own tho.... And it didn't do it regularly either.. Just a random thing. Put it down to a fluctuation in vacuum at idle when braking with the car still rolling. Idle valve maybe over compensating for the fluctuation of vacuum levels...