Same problem here, I`ve formated my pc 3 times already.
I had similar issue on my PC. Intel i7, two AMD 6790s running through displayport to 27" screen. I found it was the driver 15.7 and 15.7.1 that was causing the issue. I rolled back to Windows 7 and driver 14.xx and it works fine now.
I saw another post someone saying a hot fix was released yesterday for 15.7.1 so you could try downloading this driver again and installing it.
I tried with new catalyst 15.7.1 but nothing work, when installing video driver the screen blinks 2 times but on the 3 time goes black and the PC not work anymore (also after an hard reboot).