Do you use discord or something? So, I can hook up with you a bit faster... I mean, I wanted it fixed bro.. 3 months, and damn, I daily think about selling this thing off... coz it's not doing me any good at all... Could you please?
I don't use discord but I'm typically on Steam after work hours around 8pm or so EST. I'd gladly try and help you out then but I wouldn't ditch the card just yet. I was ready to turn mine in at one point but I confirmed that it worked perfectly on another system; the heavy power draw is what lead me to inspect my power supply further.
I think you may be onto the main problem for the low fps. The OPs PSU can, do 1000W of work total, but that work is split between the available rails by splitting the amperage. So each rail is only capable of doing 250w of work. For Vega then, it would be imperative that each of the two 8-pin cables be on a separate rail.
As for the crashing, I still think that is related to the 1.35v and 3.8 Mhz on a stock cooler on the CPU. Scaling that back should help with the crashing.
The image I have attached is "hopefully" your PSU. You want to make sure that one of the 8-pin cables connected to your Vega, is installed in the red 12V4 slot on your PSU. The second 8-pin cable should be in the red 12V3 slot on your PSU. No other 12V4 and 12V3 slots should be populated with any cables. 12V1 isn't the greatest to use for your GPU, as all your other PC components (HDDs, etc.) are powered by this rail as seen by the five pin connectors at the top of the image.
You want to make sure the two 8-pin Vega cables are going to a different 12V#, and that those rails have nothing else on them.
Looks like others had similar issues with GTA V and that specific resolution.
It was resolved by running the game on Ultra with X2 MSAA and all the advanced features disabled. Unfortunately, I do not have GTA V, and can't test this out on my own Vega system.
Forgive me, but is the bottom blue circle covering 16 pins? It looks like both 12v3 and 12v4 are filled down there and 12v4 is used again in the upper circle.