Here is the Maintenance and Service Guide for the computer. On Page 40 it states as following:
Power supply ● 800 W Gold, 90% efficient, wide-ranging, active Power Factor Correction, two auxiliary dongles on two separate 12V rails, ERP 0.5W, Built-in-Self Test (BIST)
The two auxiliary dongles (power connectors) are apparently on separate 12V rails. Therefore, it leads me to believe that you should have enough power to run the GTX 970 using the 8-pin adapter. However, since it works with some games and not GTA V, it may be that it is far more video intensive.
Here is what it states about Graphics cards:
Graphics cards
● PCIe Gen3 (PCIe3) bus speeds; can support two PCIe Gen3 graphics cards in PCIe3 x16 slots
● Up to 225 W graphics or computer card in the primary graphics slot
● A second graphics card in the second PCIe3 x16 slot
● Third and fourth 2D graphics cards in additional PCIe2 slots
● Combined power consumption of all cards cannot exceed 300 W (subject to overall system power limitations and configuration restrictions)
If you haven't done so, you should check for patches for GTA V. The developers leave it up to us to correct problems that they should have fixed before releasing the games. I have had games that would restart the computer before installing the patches.
