DIAGNOSIS This is a signal dropout. The DisplayPort or HDMI handshake is failing, usually due to a bandwidth limit or cable issue. At 7680x2160 and high refresh rates, cable quality is critical.
FIX Replace your DisplayPort cable. You need a certified DisplayPort 2.1 cable, 80Gbps bandwidth. Do not use an older DP 1.4 cable, even if it 'fits'. Ensure it's firmly seated at both the monitor and GPU. In the monitor's OSD Menu, go to System -> DisplayPort Version and ensure it's set to 'DP 2.1', not 'Auto'.
IF THAT DIDN'T WORK The issue may be with the GPU's output port. Try a different DisplayPort output on your graphics card. Also, update your GPU drivers to the latest version.
HEADS UP Using the included DP cable is best. Aftermarket cables must be VESA-certified for DP 2.1 UHBR20. A cheap cable will cause this exact problem.