Leave Fast Startup disabled. Then let it run for a while to see if problems recur.
If so, then we can troubleshoot them without worrying about the Fast Startup interfering with it.

A failure to post is a concern. Do you have a BIOS or UEFI? The more I see issues with BIOS and Win8, the more convinced I am that UEFI might be required for Win8. Did you check to ensure that no settings were changed in the BIOS/UEFI? Win8 communicates with the BIOS/UEFI much more than Win7 did.