There are four rails on this PSU, 3.3v, 5v and 2x 12v. If you have plugged the PCI-E power cable into the RED socket then that is on a 12v rail. v3 refers to the 1st 12v rail and v4 refers to the second.
The information which you have received so far on troubleshooting this issue seems to be leading you on a wild goose chase. It should be as simple as, fit GPU to PCIe socket, connect GPU to PSU, install latest drivers, reboot.
If you are having issues from there and it's not happening in games (which is a much higher demand on the machine than playing a video) it would suggest it's not the PSU causing the problem.
Have you tried a different graphics card in the computer (or tried this graphics card in another computer) and seen if the problem follows the card?