In the BIOS change the setting to "Prefer Legacy OROM" or something similar.
The new video cards (should) have 2 option roms, one Legacy, and one UEFI.
So does the MB, but the UEFI byte code for z68 does not work with UEFI on video cards. Intel did not make it work.
So you need the Legacy Option Rom boot option, which is on my Gen2 board 3xxx BIOS... should be on your Gen3, if not ask ASUS.
If you get real desperate you can try to flash and ASUS 980ti bios onto the 980ti as I think ASUS have a special bootloader on graphics cards which fall back to Legacy better.... but you would have to find a compatible BIOS and know what you were doing.
The other thing that may be worth asking ASUS is whether an Ivy bridge has better byte code in UEFI for the iGPU... since you have a Gen3 motherboard it might be better designed for the Gen3 processor. i.e. the clash in Intels code might be related to the PCIe2 logic for Sandy Bridge... I could not find much info on it though.
The other thing you need to check is whether the 980ti's actually have a Legacy OROM, it would be unusual if it did not though.
Microsofts dream is to kill off vga... so likely to be more carnage in the future. They seem to currently be trying to kill off boot managers and PCIe2 with UEFI, so it is probably evil planned obsolescence, from Intel and Microsoft.