rshwayder
I had a similar issue and something that I did solved it, I just don't know exactly what. Note that if I waited several minutes, my computer would eventually boot up.
 
- Flashed the bios on the video card (with official bios on the forums here)
- Made sure my motherboard bios was up to date (Asus P8Z68-V Pro/Gen3)
- Reset my motherboard's bios settings to default (got rid of overclocks and the like--I actually cleared CMOS with the jumper pins to do this)
- Used Display Driver Uninstaller to get rid of my drivers
- Installed the latest NVIDIA drivers in safe mode without anti-virus software
- Made sure everything worked now (it did)
- Overclocked my CPU and changed bios settings as needed again
 
Along the way, I also enabled the motherboard's iGPU and gave that boot priority, then hooked up my second monitor to that. That temporarily solved my boot issues and I was able to game on the monitor connected to the 970. However, after I did everything about, I was able to disconnect from the iGPU and only use the 970.

eox26
If you get this solved PLEASE let us know. I have this EXACT issue. In fact I've done nearly everything known to man to get this to work with my gtx 780 since last January. I've purchased new ram, new motherboard, new PSU, everything new except CPU. My gtx780 works flawlessly in the 4x slot but will not work in the primary slot in any of my mobo's. I have the asrock z68 fatal1ty motherboard and also a asus z77 sabertooth mobo. This issue is on both. At first I thought maybe something was wrong with the GPU so I stuck it in my old nvidia 790 sli mobo (I think that's what it was called..I can't exactly think of what it's called off the top of my head) and it works just fine. I also have an old nvidia 550ti that works in ALL of my mobo's primary slot so I know the slots aren't the issue. I've upgraded my PSU to a 1000w so I know there is a enough power. I've never heard of the vbios thing before and I'm interested if this does work and how you do it exactly. Sorry for the long post and good luck! Again, let us know what solves your issue. I am in desperate need to get this frustrating issue resolved!

Xuvion
Hello everyone,

TL;DR: make sure you really update your bios and do it with software of the mobo facturer out of windows.

I experienced exactly the same problem as described by the author (and many many other users on the web).
My System is as follows:

• MOBO : ASUS P8Z68V Pro
• CPU : Intel i7 2600k
• GPU : EVGA Nvidia GeForce GTX 970 FTW+ ACX 2.0
• RAM : 8GB Corsair 
• OS : Windows 8.1 64bit
• PSU : Corsair GS 700 (700W)

I tried everything to get the card working: 

• updated BIOS
• reset CMOS
• changed PCI settings in BIOS
• put the whole motherboard out of the case to see if the card really fits in the PCIe slot on it

As nothing of those things got the problem fixed I contacted ASUS and EVGA support.
ASUS just gave me the advice to reset cmos, update bios etc. all those things I already tried...
EVGA offered me a RMA for the card. (although the card worked in the system of a friend of mine with older mobo and lower psu...).
Today the exchanged card arrived and I experienced the problem again.
Frustrated, I just repeated all those things again and startet with updating the BIOS (although it already was at the newest version of 3603 for my mobo). I thought maybe it'll work and it did!
I literrally updated my BIOS from Version 3603 to 3603 again and now it works. With the first card I updated BIOS via an usb stick out of the bios but today I let the Asus AlSuite software doing the job for me. It looks like although my BIOS showed the new 3603 version and showed that the update was successful the first time, it was not 100% successful. So if you have this problem too just make sure you really update your bios!

Greetings Xuvion

_zeus2_

I got the same mainboard (Asus p8z68 v pro gen 3) [...]