I have been told that so many 360's where sent out without testing. The console heats up and the delicate soldering on some of the chips loosen up and when the console is moved the chips come away from the board and some of the pins no longer make contact. You can place the console horizontal and wrap a couple of bath towels around it and leave it switched on for a couple of hours. If you are lucky the chips will reseat themslves. DO NOT leave the console as it could cause a risk of fire. Getting your console Flashed does not cause this problem, it is a cock up by microsoft who rushed out the consoles untested. Returning your console to microsoft regardless of age for repair is your best and safest bet. They will replace the whole board and return it to you working.
Good luck
The Oracle, December 2008