I'm not looking for a solution (but if there is one I'd like it) but rather some pointers so I can investigate a bit.
2 x RPi3B running LE9.2.8
1 running TVHeadend server, cabled ethernet, Hauppague dual USB tuner, recording to the 128GB MicroSSD LE boots from
1 cabled ethernet, 6TB powered USB HDD, wifi keyboard dongle, hdmi connection to TV
At random intervals, by which I mean could happen two days in succession then nothing for a week, a month or a few months, then several on one day.
Both Pis are left running when the TV is switched off and a reboot NEVER happens when the TV is on and a film is being played from the 6TB HDD. It also doesn't seem to happen when something is being recorded. Generally it happens overnight whilst we are sleeping but has happened during the day.
Living in the Scottish Highlands my prime suspect is power - enough of a fluctuation to turn the Pis off and on but not impact on a PC being used at the time, or even since I use powerline ethernet, a signal hitting the ethernet cable.
Does anyone have any other suggestions, preferably with some way to test them before I go and spend money on a UPS?