Hi. Apparently there are some long-standing issues with power management on the BBB which may be contributing to my problems. Something to do with how the PMIC handles power glitches/brownouts/out-of-tolerance etc. I will be investigating these.
Yesterday was particulary hot here (27-30) during the worst manifestation of the shutdown problem. Later in the day with temperatures below 25 shutdowns were ocurring only after a few hours!. The correlation is interesting but whether there is a causative relationship here I don't know yet so will need some time to follow this up also.
I'll be off the grid (camping) for a couple of weeks so will pick this up on my return.
Thanks for all your help so far.