just an update for the techs - I posted about this about a month ago. It was suggested a video driver problem was at fault - and certainly using server (no screen) version fixed it - but really I wanted a screen. The graphics drivers (Nvidia) were updated recently and the problem seems to have gone (standard Roon version) with them installed. Or perhaps going from build 988 to 1105 has fixed the issue.
Thanks for the guidance.
I had the same problem on work machine, where I work without network devices. I eventually installed a Roon using a server and a client instead of Core running both functionalities.