1). You might try placing the diode between the j1-1, and j1-2 terminals, the diode will work as "reverse emf protection", and can help drain off the power stored in the motor windings when the mosfet is switched off. I see the MOSFET your using has a built in diode, but I would still put one across the motor terminals.
2). Play with the value of R1. R1 is used as a pull-up resistor, it holds the gate of your n-channle mosfet HI (OFF), It maynot be holding it high enough, try 500ohm.
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