The stop-start on my wife's Fiat 500 often either fails to cut the engine when in neutral at traffic lights, and this is accompanied by a dashboard warning symbol, or else it cuts the engine properly but then restarts the engine by itself about a minute later. Being intermittent, two main dealers have failed to find anything wrong, but it is still frustrating. The car has done about 9000 miles and is used for both long and short trips. Any ideas?