If it is not the power supply, try restoring the console to factory settings. When I had mine I kept getting the green screen and when I was able to finally hit my dash I reset the box to factory settings, reinstalled the update and it fixed it.
To reset it manually, press the power and eject button simultaneously and hold them for about 10-15 seconds. Let it turn on and then reset to factory setting again.
And if it is the brick, try plugging into a different outlet. I know that sounds crazy, but sometimes wattage from outlets will vary depending on how much power is going through your room or grid of your house.