A brushless that cuts out under load at half throttle is usually a bad speed contro(regardless of batteries). It's (sometimes) happens when the speed control fries one of the 3 phases of A/C current to the motor. Two phases will make it spin but no torque. Another big symptom of a missing phase is the motor(s) not starting until you give them a little spin...then seeming ok until they're under load.
my $0.02