DIAGNOSIS: The DisplayPort link is losing sync, often due to a marginal cable or a bandwidth overload.
FIX: First, power everything down. Reseat the DisplayPort cable firmly at both ends. In the monitor OSD, go to 'System Setup' -> 'DisplayPort Stream' and change it from 'DP 1.4' to 'DP 1.2'. This reduces bandwidth but increases stability.
IF THAT DIDN'T WORK: The issue may be NVIDIA G-Sync Compatible mode. In the NVIDIA Control Panel, under 'Set up G-Sync', uncheck 'Enable G-Sync, G-Sync Compatible' for this monitor and test. Some VG28UQL1A units have erratic behavior with it enabled.
HEADS UP: A DisplayPort cable must be rated for HBR3 (High Bit Rate 3) to handle 4K high refresh. A cheap cable will cause this exact symptom.