I think it was back in Mavericks where I initially had the problem with dual monitors. Screens would often not come on its two displays were plug in at boot. Couldn't wake up from sleep frequently if dual displays. It was finally patched and fixed. Running nMPro with dual Dell 2715q. Having the exact same problem now with Sierra. I've done the NVRAM and SMC resets. In addition, UI scaling is no longer an option. I got it working once, then can't get it back. Without UI scaling, i get lots of artifacts and media files won't play with either QuickTime or VLC. After a re-install, a clean install, countless reboots and SMC/NVRAM resets, i've given up and rolled back to ElCapitan. Its running flawless on my retina MacBook Pro '13. Last time i'm leaping to a new OS release. I had no problems with Yosemite and ElCapitan. But I think Apple just not putting enough effort into their MacPro.