2007 Eton 90 4 stroke - grinding at higher RPM's
 

I am wondering if anyone has had this same problem or know of a possible fix. My son's 2007 Eton 90 is grinding when he gives a lot throttle. It is almost like it is slipping out of gear. I checked the gears and the cam that ingages the front and back drive and everything seems to be fine when apart. The battery seems to have lots of juice, but could a battery with not enough power cause the shifting motor to disengage or is there a way to check if the shift motor is faulty? It is electric shift with a belt drive for those who do not know. Any suggestions would be appreciated? Thanks.