one way to check for worn brushes or commutator is to place the car on a stand, give it very little throttle, just enough to barely move the wheels. If the wheels stop or don't even move, keep the throttle at the present position and with the other hand turn the wheel, if it takes off running then you have worn brushes or a flat spot on your commutator.

Brushless motors are great, the only drawback is the wear factor. Brushes can be replaced, but the comm has to be cut on a lathe, which the average joe doesn't have. That is why I moved on to brushless. But both motors have pros and cons.

It may be time for a new motor my friend.