Hi Friendkiller96,
For your R9 390X I think Driver version 16.8.3 is overall working best. I know AMD made many small driver updates after that, however all aimed at RX400 series and non that I know of for R7/9 200 or 300 Series. So please try this:
Uninstall the Graphics driver and all related Graphics software like Afterburner/GPU-Z or whatever graphics related software you might have installed.
Best way to remove Driver Software is to boot Windows in Safe mode, so the registry can also be cleaned. If your system does not allow you to do that, you can also use a program like DDU to remove such software: http://www.wagnardmobile.com/DDU/
Reboot and manually download and install 16.8.3 graphics driver (don’t let auto installer from Windows do that). You can find that here: http://support.amd.com/en-us/download/desktop/previous/detail?os=Windows%2010%20-%2064&rev=16.8.3
I will have [email protected] send you an email with latest firmware for the MST Hub so you can update it.
One more question: You are using normal HDMI Cables to go from MST HUB to Screens ?