Check PWM freq,
Name: Capture19.JPG Views: 8378 Size: 73.3 KB
If too high, the inductance of the motor affects operation, add diode across motor pointing up, returns energy to motor when MOS FET switches off. Hardware PWM freq only goes so low. May need INT to PWM or do it in basic.
The ckt shows P-channel MOS but your N-channel setup is ok.

Don