DIAGNOSIS That's a classic sign of a flaky connection or a cable issue. The signal is dropping out momentarily. For monitors with AMD FreeSync like yours, it can also happen if the feature is enabled but your graphics card or game is having trouble syncing with the 75Hz refresh rate.
FIX Start with the cable. If you're using the HDMI cable that came with the monitor, try a different, high-speed HDMI cable. Make sure it's firmly plugged in at both ends. Next, go into the monitor's menu (using the joystick), navigate to 'Game' settings, and turn 'FreeSync' to 'Off'. See if the blackouts stop.
IF THAT DIDN'T WORK The issue could be with your graphics driver. Go to AMD or Nvidia's website (whoever made your GPU) and download the latest driver. Do a clean install, which will wipe old settings that might be causing the conflict.
HEADS UP If you're using a cheap HDMI splitter or a very long cable (over 10 feet), that's often the culprit. Connect the monitor directly to your PC with a short, quality cable to test.