benelli 491 sport missfire?

my sons benelli keeps missfiring. it starts ok but if you hold throttle so tick over is just reving you can here it cut out but recovers just before it dies. If you ride it every now and then it missfires but closing the throttle alows it to keep running and off it goes again.
i have had carb in bits to clean, air filter clean, new plug and new battery but it just comes back again.
i think it could now be electrical.. any ideas please
Could it be auto choke???? or cdi breaking down???