I'm having this trouble too. Mine is the new 4K model, powered through a USB3 hub. I've tried 65w chargers, 5v 20w power supplies (constant voltage) with no luck.
Eliminating the USB hub doesn't seem to change anything.
TV will turn on and I can cast to it from my phone, but no signal.
Reseating the hdmi does nothing, not a flicker. Power cycling the device with the cable fixes it every time. But it happens daily, sometimes more. I need a fix.