Thank you for your answer.
As it happens, the bug stopped some time after I reloaded the grub files a few
weeks ago.
Since then a number of patches have been edited which may have also corrected
the problem.
I really don't know what happened but now I can enjoy Ubuntu without fearing a
Public bug reported:
many errors in the syslog file
ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: xorg 1:7.7+13ubuntu3
ProcVersionSignature: Ubuntu 4.10.0-42.46~16.04.1-generic 4.10.17
Uname: Linux 4.10.0-42-generic x86_64
.tmp.unity_support_test.0:
ApportVersion: 2.20.1-0ubuntu2.14