Resistance test means little when you are only throwing milli-amps through the motor with a meter. I grind down a commutator stone to fit in the brush holder, release the brakes so I can push the truck around, and then roll it back and forth to rotate the motor and clean the commutator. Do that, and then try the truck.
Look inside the brush holder after you clean the comm. If you see a burnt bar or two, you need a new motor. If it runs for a week or two and the problem comes back, you can will probably see the burnt bar then if you did not see it when you cleaned the comm.
Tonymfg ideas should also be followed.