I've had the same problem. Got my Mac Pro in May 2016, running mainly ProTools 12 with Apollo Thunderbolt Interface, OWC TBolt hub.
Within a week had the issue: screen freezes, audio continues, can't exit out of any software so I force quit. I had been running the same system on a Mac Mini with none of these problems (unfortunately it just was not powerful enough).
Console lists repeatedly: GPU Driver appears to be hung (over 5 continuous seconds of unreadiness.)
Troubleshooting included no internet, running one monitor, disconnecting OWC TBolt hub, adding more RAM.
Brought it into Apple last September and they replaced the Graphics Card A. The problem re-occurred and I did some diagnostics with the Apple Guy on the phone, which reduced the incidences for a while but they promptly returned. (These were: Reset System Mgmt Controller [unplug all connections and hold down power button for 15 secs]; Reset NVRAM [start holding down OPT COMM P R keys]; and reboot in safe mode)
I just got it back from the Apple Store (for the second time) and this time they replaced both Graphics cards as well as the Logic Board, and it's frozen twice in the hour since I booted it back up, straight from the Apple Store.
Oddly enough, they said they needed to erase the computer to run a full diagnostic but they did not erase it. They were very insistent on erasing it in order to properly diagnose it...
Has anyone been able to resolve this problem?