Hi Dave,
From your description I think what is going on is "single phasing" of the motor. This occurs when the motor looses power to one of the leads. There are a number of things that can cause this but since you flew the plane 4 times without issue my guess would be that a solder joint has failed. It could be on the ESC, the connectors or at the motor its self.
So you have 2 options; because it is a new model you can return it and have the power system repaired or replaced under warranty or try to repair it yourself. Just bare in mind that if you do it yourself your warranty will be over. WJ.