I think I have found the cause of my random system resets, nothing at all to do with software, there was a whisker of solder or something similar adjacent to the little PCB which has the reset button on it and it occasionally reset - probably with thermal cylcing or whatever.

I found it because the reset button wasn't working and, getting it to work, I found that wiggling the board caused a reset.

Humph! There I was thinking it was some wierd option I'd set when building my kernel! :-)