I have the Slash Platinum edition, using the stock plastic center diff. I have installed the Traxxas aluminum motor mount #7460x using the stock 10x15x4 bearing. That bearing was replaced when I installed the new pinion and spur gear. How ever that bearing failed 3 runs after I replaced the 2nd spur gear and new pinion
You must have another failure somewhere else in your drivetrain or wheel bearings, the issues you're experiencing aren't normal.
Slide the motor away from the drivetrain and see roll the car to see where it's binding.