On further reflection, the obs is currently on the same wiring circuit as a rather large automatic water pump. I think the probable cause of the runaway start could be the resultant voltage drop/spike, thus initiating the Gemini to revert to factory defaults and Cold Start.
Gemini to be removed, remove/discharge CMOS and remodel/start again!
And of course, an AGM battery now to be fitted to the obs with a 12v trickle charger to stabilise/buffer the input voltage to the Gemini.