Probably something is overheating.
Check the temperature of the driver chip; you may need to add a heatsink and/or cooling fan.
Also check the temperature of the motor itself; it shouldn't be too hot to hold your finger on it. You may need to adjust the current going to the motor down a bit.
Check the temperature of the driver chip; you may need to add a heatsink and/or cooling fan.
Also check the temperature of the motor itself; it shouldn't be too hot to hold your finger on it. You may need to adjust the current going to the motor down a bit.