I too have a P9X79 Deluxe motherboard. I was finally able to install 8.1 from a complete reinstall of Windows 8.0, something I was resisting for a long time. My point though is I reinstalled with whatever Version 8 had to offer in drivers - non of my own Asus,
NVIDIA, Intel, etc. drivers were used. I also disconnected all non-essential peripherals before the reinstall. Once Win 8 was reinstalled, I installed 8.1 (as an update) without a problem. I then hooked up all left-out peripherals and installed all my own
drivers.
I agree, its a pain, especially when you have a boat load of programs that have to be reinstalled afterward, but there is something to be said about unloading accumulated junk and non-essentials.
Anyway its over for now and it worked. Hope this gives you hope. Until next time.
David B.