Quote:
Originally Posted by lgetty
My iMac 5k had some odd video behavior until I went ahead and did a fresh/clean install of Sierra.
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This is, what Robert should test ... install the latest Sierra on an external USB HDD or SSD and check, if the same behaviour is reproducable or not.
I did such a test before upgrading to El Capitan ... made an USB-stick OSX installer and installed from that to an external SSD and bootet from that to check if everything runs as expected.
If the test is successful a fresh install is the solution. I wouldn't be astonished, if that helps, because I often found issue reports about Yosemite and El Capitan, that mainly occured with upgrade installations, but not with fresh installs.
I also wouldn't be astonished, if Justin's Apple Store test will be issue free, because that iMac will likely run a fresh install of macOS.