if one motor seems stronger than the other you can adjust the "balance" (with handset) to fix the problem.

Crown says it might be the brushes not setting correctly. And there fix is to put it into tow mode and push the wav back and forth 10 times to seat the brushes.

crown had a batch of brushes that went into new motors as well as replacement brush kits. The compound was too soft, if you replaced the brushes do it again. Make sure the new ones has little "sparkles" in them. stone the comm. well and thoroughly clean out the carbon dust!

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.

Is possible with a faulty controller. Rule out dirty armatures, low brushes and calibrate pots. Use of handset will sure help you out alot.