x reg sirion auto gear

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i have a little runner sirion x reg on auto.
when its on the road and stops to wait for traffic the engines cuts out if gearbox is on N.
I kow keep the gearbox on D while stood on traffic but still the engines seems to cut out then comes to life.
If i park the vehicle and change gear to P the engine cuts out soon after.
When i purchased the car it did not have this but not it has.
please advice.

ben