Any ideas what could be causing this? Also is there anything I can do to make this driver even tougher?
Additional info: The motor is speed controlled by a microcontroller. The PWM duty cycle is capped around 60%. The motor can draw about 5A or more depending on the load, but this whole thing is powered by a switching power supply rated for 3.75A with a (fast) over-current shutoff, so that should limit the current in cases with excessive load. PWM frequency is approx 700hz.
I don't know what's burning out first, the mosfet or the diode. I tried replacing the diode (before I knew that the mosfet was also burned), and it burned out again after I toggled the interlock a few times.