N-MOSFET motor driver full protection
First, your motor inductance with 1 uH is totally unrealistic... A more realistic value would be in the milihenry range.

Second, I think you problem is that you have an overvoltage condition in the drain to source voltage. Your gate seems well protected with a TVS diode and a 100 ohms resistor in serie with the gate. However, you need a fast recovery diode across the motor. When the transistor will turn off (or become in open circuit as others prefer to say), the current trapped in the motor inductance must be able to flow somewhere and this is where the fast recovery diode come into play. In your range of voltage, a schottky diode would be perfect to be placed in anti-parallel with the motor. 

If you don't do this, the high inductance in your motor will cause a big overvoltage condition than can ring well over your maximum drain to source voltage (30 V in your case). You may think a TVS diode could shunt the overvoltage but, in order to do that, they must conduct a lot of current which TVS diode are not made for. This is why a shottky diode or a fast recovery diode are much more suited for your application.

I hope this answer will solves your problem!