DIAGNOSIS Display signal is dropping. This is almost always the Adaptive-Sync/FreeSync handshake failing or a weak HDMI/DP cable.
FIX First, disable FreeSync in the monitor's OSD menu under 'Game Settings'. Test. If blackouts stop, the issue is with the sync signal. Update your GPU drivers. Use a certified high-speed DisplayPort 1.4 cable, not the included HDMI. In Windows/Nvidia Control Panel, set refresh rate to 240Hz manually, not 'auto'.
IF THAT DIDN'T WORK The monitor's mainboard T-CON (Timing Controller) may overheat. Ensure vents are clear. The symptom points to a failing component on the board regulating the high refresh rate signal.
HEADS UP Using a non-certified or long cable at 240Hz will cause signal dropouts. The bandwidth requirement is extreme. A '4K' rated cable often works for 1080p 240Hz.