Updating Bios on X370 Killer SLI/AC

well im glad we got to the bottom of this

yes technically 3900 is from version 5.4 up

i would suggest 5.8

for the reasons of

you board is virtually the same as my taichi
im on 5.8 and its stable and fucntional


now one tip i must pass on

when i bought my 3800 i flashed bios to 5.8 using my 1700 cpu


all working fine
i then installed the 3800 and it wouldnt boot to bios

i finally got to thebottom of this by using JUST ONE ram stick in a2 slot
and it booted to bios , i could then switch off and insert 2nd stick in b2 and reboot set the xmp for ram etc

after this if i ever have to clear the cmos (reset bios )

i always have to use JUST ONE ram stick for the very first boot


i mention this as our boards are very very similar and you might have same issue