I'm having similar problems with this poster (except he's on the X370): http://forum.asrock.com/forum_posts.asp?TID=4592&title=x370-killer-sli-cant-get-into-bios

I'm 99.9% sure that it's related to ram. Things were fine -- meaning I could hit DEL and get into the bios every time -- until I tried to get my Corsair CMK16GX4M2B3000C15 to get higher than 2666MHz (it's rated at 3000). This appears to be pretty common with that series, according to many reports on the net. And yes, they are on the QVL. 

In short, what happens is that after I made the change to the ram frequency and reboot, hitting DEL or F2 no longer gets me into the bios. As mentioned above, I had this same problem with the previous board above. What's interesting is that this board will boot. Where I'm screwed is that I was trying to set up clover to dual boot, but I can't change the boot drive to boot from USB. That plus all the other things you need to do in the bios.

This isn't hibernation (it's off)

This isn't fast boot (That's also off, but I'm also using Restart, not Shutdown)

I've tried different sticks of ram. I currently have a G.skill Trident Z that's also on the QVL

I'm on the latest bios version 1.60 

Where am I screwing up? Any ideas on how to fix? With the previous MB, I managed to get the bios to start 1 out of 10 by just restarting and mashing del. Maybe I'll just try that and hope that I can get in once and lower the frequency.

Any other recommended approach(es)?

Thanks