Created attachment 113945 [details]
syslog

Hello,

Using an Apple Cinema Display (Thunderbolt) for a monitor. Booting and monitor detection works fine. Suspend/hibernate never works (that's another issue I'll have to post about after solving this one). Two AMD GPUs (AMD FirePro D300s, 2gb RAM each) on a MacPro (dual/tri boot with Mac and Windows). 64gb total memory.

After a varied amount of time with each occurrence enjoying Kubuntu 18.04, my display randomly goes black. The kind of black when a monitor shuts off/unplugs (no backlight, no mouse, etc.) The keyboard and mouse (which both have LEDs) seem to be powered on still (caps/num lock LEDs work, mouse DPI LEDs, etc.), and they are plugged into the back of my thunderbolt display, so I'm not sure if the actual system is still running and/or my display is actually at fault. Hot-plugging the thunderbolt display doesn't solve the issue. Once I was able to plug in an HDMI TV as a second display and it somehow made the thunderbolt display re-appear, along with the second display. That hasn't worked since.

I checked the syslog for errors when rebooting, and have attached the output. Looking at the log, my guess is that my GPUs are playing nice with the proprietary AMD radeon driver. I've also attached the output of lspci | grep vga for reference. 

The error is reproducible, but not on command; it happens randomly, and sometimes doesn't happen during a session.