I think I would go to the basics ... it could simply be dirty carburetor ........ or ..... when the engine stops .... immediately loosen the cap of the gasoline tank ... yes the vent is clogged or the aeration of the tank does not exist or has failed until the vacuum of the tank does not let more gasoline down and goes out ... when it loses the vacuum it starts again ...
if that is it ... verify that it exists and is uncovered. and if there is a failure of the above mentioned by Abner Flor ... take her to the agency that is under warranty, if she has only 40 km they would have to fix it ... LUCK and comment what happened