FWIW, I found that changing the BIOS setting for “iGPU Configuration UMA mode” to “UMA_GAME_OPTIMIZED” away from the auto default improved stability and got rid of the occasional screen blanking and freezing I was seeing on my 7840. Your problem sounds more severe than what I was seeing, but it may be worth a quick try before sending it back if you can get the thing to start up again.