>>Further diagnosis revealed the MOSFET is not turning fully on; given a
>>stable 5V gate voltage, it still reached 440C at 120C ambient.  Given
>>the drain voltage of 14V, it was happy and ran cool; 270C.
>
>                                            ^^^^^^^^^^^^^^
>
>THIS IS COOL???????
>
>Calvin



That was my reaction, I was thinking it had to be a misprint.
If it's real, something's still very wrong.
Is the source at zero volts?

270C is also AFAIK, WAY beyond the thermal limits!