I have an LG 27UD68 that used to run 3840x2160at 60Hz prior to High Sierra update. It now only displays at 30Hz and the resolution says 3840x2160 (low resolution) in system preferences. I can not change the refresh rate as it is greyed out at 30 Hertz NTSC. The monitor is connected Thunderbolt 2 to Display port out of my 2013 Mac Pro. Anyone else have this issue?