Apparently 30 fps cap solve the issue here, but I don't buy a 970 to play at 30 fps ¬¬
Will try the fan trick
As an alternative to the fan trick, I've also found that underclocking the card by 100MHz on GPU and mem speeds (with EVGA Precision X) fixed the problem. I didn't see a noticeable performance drop btw, and to me it's a better solution than all that extra fan noise.
So it's not overheating as such that's causing it (the card was only running 2 degrees cooler with the underclock) as I previously thought, more that the factory overclock isn't stable under the extreme loads this game causes.