What Kernel are you using? I've upgraded to the letux 4.8 RC5 via aTc's repo update and noticed this behavior with my bench supply capable of delivering 4A.

5.1V - Some instabilities HDMI, SATA resets and USB devices blink out. Sometimes freezes before getting to X.

5.2V - Boots up Consistently. random Freezes when using Firefox when attempting to watch a Youtube video. Sometime just HDMI blinks out occasionally. 

5.3V - Seems stable running CPU intensive things. Firefox running youtube video, running 720p video in smplayer and quake 2 at once. highest current draw observed ~1.6A during spikes. Random Freeze occurs generally within 5 to 10 minutes of use. 

5.4V - Same as 5.3V

5.5V - Same as 5.4V

5.6V - 5V LED blinks, HDMI blinks out or won’t go into a resolution above 1024x768. Can seem to use firefox, youtube videos without issues. 

5.7V - 5V LED is off,  but turns on when drawing  ~1A. HDMI working appropriately.   

5.8V - Same as 5.7V

5.9V - 5V LED is off,  but turns on when drawing  ~1A. oddly the 1024x768 bug appears again at this voltage. powered off a few times to confirm it. 

6.0V - 5V LED is only displays as on with current draw is ~1.4A. Seems stable otherwise. 

6.2V - 5V LED is only blinked a few times, when the current draw briefly spiked about 1.6A. seems just as stable. 

6.3V - 5V LED off completely. No noticeable blinks. appears stable.