An electric motor, specially a DC wound motor, is a lot like a good sled dog in that it'd rather die in the sled harness than tell you it can't do the job. I'd look at two areas that could be causing the motors to burn up. The first would be to low of an RPM on the motor which is giving you peak torque but also causing amps to stay on the higher side. The other, would be the load is just to big for the little motor to chug. Not sure if you can do a gear ratio change up (or deal with a slower ghost but changing gear ratio will affect motor amp draw. Adding a blower/fan cooler to the motor will also increase duty cycle/motor life as even class H insulation has a heat rated life span. Hope this helps.