There was nothing useful in /var/log/{messages,syslog}. Xorg.log had endlessly repeating lines shown in my first message that started this bug. They repeated exactly when screen was blinking.
-- Closing and opening lid sometimes leads to screen ENDLESSLY BLINKING https://bugs.launchpad.net/bugs/368946 You received this bug notification because you are a member of Ubuntu Desktop Bugs, which is subscribed to gnome-power-manager in ubuntu. -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs