Quick reply for the guys struggling with constant random reboots ...
I've tested a 5950x on an ASUS Crosshair VIII Hero in the following configurations:
All settings not mentioned at left at default values
Mem @ 3600Mhz C14
Fmax Enhancer Enabled
No issues or reboots experienced
Running the same CPU installed on a Crosshair VIII Dark Hero i was experiencing random reboots during idle / light load straight off the bat. I started looking at which settings could be causing this in the BIOS and found that disabling Fmax Enhancer immediately stopped the random reboots.
I haven't spent any time trying to figure out why this is happening, but i would suggest if you are experiencing the reboot issue, to disable this.
Having PBO enabled didn't affect the rebooting, it was only the Fmax enhancer.
Best of luck to you all out there, chances are there's nothing wrong with your CPU and it's something to do with power delivery and Fmax enhancer.
May the force be with you !