I later found the true solution to the fault: the connection to external and internal power buttons. I disassembled and applied a small amount of strong epoxy resin to the extrusion on the underside of the external power button. The device has encountered no issues for some months now. Disassembly demonstrations are available on YouTube.
I simply struck the back, bottom edge against a wooden surface 4 - 6 times vigorously and the Kindle (5th) unfroze. I tried hard resetting. No joy. It was the last resort. Carry out at your own risk. I believe the problem was caused by accidentally leaving the unit atop a speaker.