Hello,
to put it in simple; the old motors have a design fault. What ever you do you'll end up replacing them.

I've replaced at least ten of this motors with new NEO model. The symptoms are always the same; WAVE pulls to one side, motors have a loud wheezing sound (brushes jumping), steering and accelerating is twitchy, braking is bad, motors draw more current, batteries discharge quickly and of course, there is an "LEFT OR RIGHT MOTOR OPEN" error.

When you open the motors you will find a handful of brush dust and that some fields of commutator are uneven or burned. This shows that there is an issue with specific windings or the commutator itself. You can take the rotor out and machine it, but the problem will recur. If you repeat that it will only recur sooner.

Once I even gave it to a specialist to rewind the whole rotor and to solder of the commutator and check it from below. He found nothing and the problem recur again.

I should say something is "undersized" or the brush dust creates a short circuit. What ever you do you can only bring it to factory condition, which in term leads to not being good and doesn't fix the problem.

Because of the above, Crown launched NEO motors. They look the same from outside except they have a few vent holes. I didn't dare to open them because they cost a fortune. I have WAVE's running with them for a few years with no issues. Some of them used only 1 mm of brushes which is remarkable. Note that you have to change some parameters in the controller when you change them.