Any chance that the circuit has been overloaded all along and you are getting a severe voltage drop at the motors? That would explain the overloading of even the larger motors, because they would have been drawing MORE current and causing even a greater voltage drop than the smaller ones. If that is the case, increasing the motor size is going to make the situation worse, not better! How are the motors fed, i.e. wire size and routing distance. Any junction boxes in between where a bad termination might be causing a voltage drop under load?
