I would like to work with you on this issue as I was one of the original owners of the 390x shortly after it released back in June. I stumbled upon this dx11 issue when I swapped out my PowerColor PCS+ R9 290x for an MSI R9 390x for FFXIV: Heavensward DX11. I do not have GTA V so I cannot comment on that however, from my own testing and observation the broader issue here appears to be related to DX11 titles + the 390/390x + any of the current 15.x drivers that support the R9 300 series cards.
I have already duplicated the issue on 3 different MSI 390x cards on 15.15, 15.20, 15.7, 15.7.1, with the only exception being 15.8 beta (I didn't test the latest beta). I tested on 3 different desktop computers, one with Win 8.1 x64, one with Win 7 x64, and another Win 7 x64. The difference between the 2 Win7 computers is that one is intel i7 based, the other is AMD FX8350 based.
Power supply is not an issue as these computers have XFX XXX Core Edition 850W single Rail or Corsair RM850 series psus. Also, when I revert to the 290x there are no issues. When I install a gtx 970, there are also no issues.
GTA V isn't a very good title to be trying to troubleshoot as that game is notorious for stability issues across both GPU vendors. I strongly believe that if we can fix the dx11 crash issues on the freely available FFXIV: Heavensward benchmark, we'll solve most of the major concerns.
Again, here is the video that demonstrates the major problem here that most users are complaining about.
Here is the link to the benchmark if you want to test it yourself.
I currently do not have the 390x card anymore (haven't for over a month) but I will consider repurchasing it from my local Microcenter if you want to pursue investigating this issue further.
Am running an MSI R9 380 @ 1.1GHz/6Ghz and all of my D3d11 games run fine...Win10x64, build 10532, 15.8 beta's. I am running one monitor--not three. The way this guy's game is freezing when running two gpu applications reminds me of overheating GPUs...which *all* freeze when they get too hot (nVidia GPUs do the same thing), and their protection circuitry activates and they drop picture & MHz until they cool enough to allow normal use again--a few seconds, usually. Also, if you run more than a single voltage regulation program at the same time--say you are running Afterburner & another program capable of manipulating voltage, it's possible something nasty is going on there, too.
This video presents nowhere near enough information to determine whether it's a driver bug or something else entirely--which is the main problem of troubleshooting. Often people think they "know" things they really don't...and, etc...;) Benchmarks typically are meant to stress out GPUs and do not assume other software will be running at the same time--but again, I don't have freezing/black screens at all, in any games, DX11 games included. It could be there is also some conflict between the two pieces of software he's running...what about power supply?...if he runs another D3d11 game (eg, Witcher 3) can he recreate the same issues? (Doesn't sound like it.)
There are simply lots of good questions that need to be answered before we can assume a problem with the GPU before we assume a problem with the FFXIV software...
It is not a hardware problem. As I state in the video I duplicate the same problem on 3 different MSI R9 390x cards. If i remove the 390x and install my old 290x I do not have any problems. Also, if I install the gtx 970 I do not have any problems. It was already acknowledged by AMD as a driver problem in another thread so the video did prove it's intended purpose in raising awareness of these driver issues.