Well it might be that since you are getting better at drifting if is heating up from you flooring it around those corners to make it drift, so how good are you at drifting ( I'm asking to suggest a more capable motor.)

OR

If you want to not buy a new motor here are things to cool it down Heat sink, Larger Spur Gear, Less traction tires (like ABS), heat sink with fan!(heres a image)