DIAGNOSIS Your graphics card or cable cannot reliably handle the bandwidth for 1080p at 120Hz with Adaptive-Sync enabled. This is a bandwidth or signal integrity issue.
FIX Open Windows Display Settings. Lower the refresh rate to 100Hz. Test. If flickering stops, your setup can't sustain 120Hz with variable refresh rate active. Keep it at 100Hz for stable operation.
IF THAT DIDN'T WORK Disable Adaptive-Sync/FreeSync in the monitor's OSD menu under Gaming. Set refresh rate back to 120Hz in Windows. If flickering stops, the issue is with the adaptive sync handshake.
HEADS UP This is often a cable problem. Ensure you're using a high-speed HDMI cable (HDMI 2.0 or later) or a DisplayPort cable rated for 1080p @ 120Hz+.