SYSTEM:
Maximus VI Hero
Sound Blaster ZxR
I7-4770k
2x GTX780TI SLI
Samsung SSD 840 EVO

History:
The initial computer set up was with a single graphic card and it ran perfectly without any problems. After approximately 2 months I have added the 2nd graphics card. The system ran well for a few more weeks before the Sound Card could not be detected all of a sudden. Reinstalling the drivers fixed the problem for the first time. Since then this reappears every few weeks. It alwas appeared when I switched to or back from SLI mode, or the computer went into hibernate mode. After deinstalling the Nvidia and Creative drivers and playing around with BIOS settings, dis-/enabling the onboard sound card or changing the boot settings fixed it and the Sound Card had been detected. But I think that was just random luck since I could not reproduce it.

Last week I had the time freeze bug with the board. I fixed it with a BIOS flash update which worked well. But after the update the Sound Blaster card did not get detected anymore regardless of what I have tried. So, I went back to the stable initial set up. Took the 2nd graphics card out and relocated the sound card into the original PCI slot I have used. Restarted the computer - surprise - it has been detected and worked flawlessly!
Next step, I relocated the sound card to the previous PCI slot. Restarted - worked.
After putting the 2nd graphic cards the sound card had been detected for the 1st start after that no more.
Finally, I went back to the stable single gpu setting and - surprise - it worked again and since then!

Unfortunatey, when I have 2x GPUs onboard there is just PCI slot for the Sound Card left. So I can not test a different PCI slot with 2 GPUs

Problem?
- Since the sound card (hardware) is running smooth I think I can exclude it as a source
- Creative drivers?
- Defect Board?

My next step would be to exchange board with Z97 chipset instead. Does anyone have another idea of what I could try or what it could be?

I appreciate any assistance. Thanks in advance!