Just throwing in my experience here. I had this issue with my GTX 970 SC for months, tried to resolve it by underclocking, changing settings in games etc. It seemed that certain settings or specific effects in games seemed to cause it to happen (like whenever an objective was captured in Heroes of the Storm, or a very specific monster in Dragon Age: Inquisition). I got an RMA for my card (since I saw threads like this) perhaps back in May. Pulled out my old card, put in the RMA card and.... got the black screen shortly thereafter with the new card. So I thought, well, let's just try out SLI now that I have two cards. I was so enamored with the performance boost that I just decided to keep the second card, figuring that since my RMA'ed card was doing it too, it wasn't a hardware issue and I'd figure out a resolution at some point.
It had stopped for a while, but is now doing it in Final Fantasy 14 pretty reliably when I run it in DirectX 11, but not in DirectX 9. And it doesn't do it at all in the Witcher 3, which I have set to all Ultra, and runs both my SLI'ed cards at 90% the whole time. For me it doesn't seem like the amount the cards is being worked has anything to do with whether or not it freezes. I downloaded Sattelite Reign, a little top down indie game, and it froze. My experience seems to be, as I said, that certain effects (in my experience it seems like lighting or particle effects) cause it to happen. This would make sense why DirectX 9 would not have the issue with DirectX 11 would, since 11 allows for a lot more lighting effects.
I'd try the 70% fan solution, but that just doesn't seem sustainable, as it makes my computer sound like a jet engine, and it seems like a more properly functioning auto-fan should do the same thing more effectively. I've tried underclocking my cards to stock speeds nad it does help, but not 100%. Considering EVERY thread I see about this issue, seems to be with this particular card (GTX 970 SC), it seems like just a really significant faulty run that is really common. I'd RMA them again, but my experience with my first RMA didn't go so well. I would swap these cards out for stock non-OC cards in a minute, since they seem much more stable... do you think EVGA would do that for me if I told them my situation?