Thanks. It's a V6 running Tivo - hence posted in the V6 board.
I'll check the diagnostics as you mention (short cut for others Home button and 9 on the remote) as I hadn't spotted the Ch998 pattern - that will be helpful, Thanks.
Re: Power Saving. The box is "Forgetting" the Power Saving settings I have set (I don't want the default setting, so change it each time) and the Power Savings screen comes up as you go into Help and Settings - this seems to be the sort of behaviour on first boot to push the "green" credentials of the V6 towards the user. All logins for the streaming services are also forgotten, but no other settings.
Anyway, I'll check the tuners, as suggested, next time it happens to see if there's been a restart.