Diagnosis
Display signal is dropping. This is almost always the Adaptive-Sync/FreeSync handshake failing or a weak HDMI/DP cable.
How to Fix It
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.