-- 3 Weeks Later --
1. When I thought my Pi was down it was only the WIFI that had stopped working (and no blinking of the USB WIFI dongle).
2. The steps I outlined below didn't work, longer term.
3. I swapped my generation 1 Pi B to my new gen 3. Still up: 2 weeks without a failure and without the steps listed below. However, its WIFI dongle is different. I need to swap them to see whether the problem is the dongle.
This didn't help (nor hurt):
I have a Pi powered through UPS that is programed through cron to monitor 2 Particle Photons that are doing real work. But, even though scheduled tasks happen every few minutes the dumb Pi seemed to go off line (sleep?) after a day or so. So I went looking for voodoo incantations to correct this problem. Here are 2 that I tried:
In /etc/kbd/config changed the line--
POWERDOWN_TIME=30 to value 0 (= never)
In /etc/lightdm/lightdm.conf after [SeatDefaults] added--
xserver-command=X -s 0 -dpms
One (both?) of these seems to have worked. 'No idea which -- or if they are black magic.
Pssst! Linux is a mess.