Some further information from some investigations:
Firstly, following up on the "time theory" above. It seems to occur at times xx:03:07, xx:08:07, xx:13:07, ... Ie, every five minutes at seven seconds past. This seems very regular.
Secondly, following the comment above, I had been previously running LibreOffice-Calc, but the problem persists after it was shut down. (Confirmed with a "ps -ef" command that there were no stray tasks.
In any case, I would think that whatever LibreOffice does, gnome should not hang like this!
Some further information from some investigations:
Firstly, following up on the "time theory" above. It seems to occur at times xx:03:07, xx:08:07, xx:13:07, ... Ie, every five minutes at seven seconds past. This seems very regular.
Secondly, following the comment above, I had been previously running LibreOffice-Calc, but the problem persists after it was shut down. (Confirmed with a "ps -ef" command that there were no stray tasks.
In any case, I would think that whatever LibreOffice does, gnome should not hang like this!