OK, happened again. This time it is connected to a different power outlet with no surge protection strip - I just got a new one, this one Belkin, and have moved to that. But the last outage happened last night before that one was installed.
It has run without being switched off for several days now. Yesterday I go out of the room, come back in and see the Gigabyte screen. Its not the boot screen, its the one immdediately before that which offers you various bios configurations. On a usb keyboard nothing has any effect here.
So I hit the reboot button and then it offers several alternative boots - this is the main board, not Debian or Grub.
I didn't pick any, and away it went, started normally and is still running fine. Unfortunately did not write them all down, but I think it showed several failed boots and invited one to choose the last good one.
I am now connected to the new surge strip which has all kinds of awesome sounding but incormprehensible parameters, and am guessing that if this happens again on the Belkin surge protection strip the sensible thing to try next would be replace the main board?
Does that seem right?
The two funny things about this were (1) that the computer was doing nothing at all when it happened (2) other machines in the house that are also running on the same ring main are not affected, so it surely can't be a surge in the mains supply. Which seems to leave the main board as the next thing to try.
Weird.
Al