CSM Causing Me Problems

[floodlitworld]

i7-6700k / Asus Z170 Pro / MSI GTX 970 / MacOS 10.12.6 / Sys Def 17,1

 

Monitors:

Primary: LG Ultrawide (Display Port)

Secondary: Acer 1080p (DVI-D)

 

I'm having an issue recently with my dual display setup. If I boot with CSM enabled in the BIOS then only my secondary display works until the OS loads. If I boot with CSM disabled, then the boot screen/BIOS shows correctly on my primary display, but once the boot process hands over to the OS (when the loading bar reaches about half-way) then the video output dies and both of my monitors stay blank.

 

Any ideas? Obviously I can work with the wrong monitor handling boot, but I'd prefer to have everything work properly.

 

Thanks in advance.