2003 Aprilia rs 125 ?

Hello my son can be riding along on a straight road at a steady 50 mph when suddenly the bike will cut out. Sometimes it can run okay for a couple of days before it happens. Other times it can be a few times in the same day. When it does happen,if he trys to start the engine again he finds it has no power and has to rev it high to pull of in first or he finds it cuts out. Iv checked and changed fuel filter in the tank but still have the same problem. CAN ANYONE PLEASE HELP ME WITH ANY IDEAS AS TO PROBLEM, THANKS