If you had another installation of the software which you'd had running in
your Mac prior to the upgrade to this newest one, you could restart the
Mac in that to see if the issue was this new macOS/OS X. To have/had a
partition on the hard drive for the new install, as a trial balloon to see if
it was going to work out for you (& your mac) that usually is best.
For the purposes of troubleshooting, you could prepare an external hard
drive of sufficient specification to be used to re-install the older OS X; or
if there is adequate un-used free-space within the internal HDD, you could
partition that drive (using disk utility from OS X Recovery) and be sure it is
correct. However if you try this, there is a fair chance it may mess up what
now is running the computer. If not done correctly, & with adequate backup.
A backup of pre-upgrade, to revert to in such cases as this, or clone of the
former complete installation; (a great clone may also have the Recovery
partition with its own OS X Utilities, etc) would be a sign of preparedness.
Sorry that I've nothing much to add; hopefully someone who sees this can
reference something else (not sure if there were any firmware updates) to
try. If the Mac does not like the latest/greatest macOS/OS X, then revert.
Or take the computer in (where applicable) to see a Genius via appointment
reservation to see if they can make heads or tails of this situation.
Good luck in any event...! 🙂
