This is a bit of an afterthought but I wanted to add that since you've added the brushless setup, the problems I mentioned in my last post are likely due to the increased power and torque generated by brushless motors. If you had your car adjusted for a brushed motor, especially if you had a lower power battery pack, you wouldn't have noticed excessive slipper slip or a slipping differential. Now that you have a more powerful motor, ESC and probably a higher output battery than before, more torque is being applied to the gears. Remember, the slipper is used for torque control and can save your transmission if used properly. The differential may have been too loose to begin with.