Hi, i´m quite new mac user. I noticed horizontal artifact on my mid 2011 (HD6770) 21.5" iMac screen on startup. I uploaded video: The artifact shows up at 0:29 on video. Also screen goes off and on, this is also new behavior after NVRAM reset. Is this normal? iMac works nice, no artifacts on normal use. Only couple seconds on startup, everytime. Problem started after I upgraded to Macos sierra and cleared NVRAM.