Had the same problem with my z77x ud5h. Looked around and found an answer: you need to update to a bios that has "Improve VGA compatibility" or any above it. After that it booted without any single issues. One more thing: if you leave your pc alone on the splash screen it will still boot eventually, just take a long ass time. Your best bet is to take out the 970, put in whatever gpu you had before to flash and then put in your 970.