I have not been able to use this E30 for a few weeks. Tried it again a couple of days ago and the screen-blanking seems to have stopped. No changes to h/w, drivers or connectors. So I'll keep my fingers crossed, and saying thanks again for everyone's help.
PS. Reverting briefly to the beginning of this thread: I also have a new TX120S3, same nominal chipset, precisely identical E3 CPU, but the TX120S3 does have on-board graphics. This means nothing apart from highlighting the benefit of fixing the configurator ...