Nereus
 
After a number of efforts it seems I have stopped the brief intermittent black screens and everything is running fine. I don't think it was ever an issue with the PSU for me. 3090 FTW3 Ultra with EVGA SuperNOVA 850T2. I will soon be switching to a 1000T2 when I build a full water loop - just waiting on a GPU block (and may be waiting a while unfortunately).
 

Nereus

jankerson

Nereus
After a number of efforts it seems I have stopped the brief intermittent black screens and everything is running fine. I don't think it was ever an issue with the PSU for me. 3090 FTW3 Ultra with EVGA SuperNOVA 850T2. I will soon be switching to a 1000T2 when I build a full water loop - just waiting on a GPU block (and may be waiting a while unfortunately).

I tend to think most of it is software/drivers other than the complete shutdowns, those are the PSU.

Sounds about right. Or the DP cable.
 

Drumjod
Not recommended (essentially doesn't work):Corsair CX750M with EVGA RTX 3080 FTW3 Ultra.
 
All hardware was triple checked including the GPU to make sure all connections were secure and HW was tightly seated. PC seemed fine at first running 4K 120 fps on an LG CX HDTV and simple games ran okay in very short tests. Only tested a heavy load using Final Fantasy XV. Experienced a hard shut down (couldn't turn PC back on for 5-10 minutes) after flying the Regalia for about 10-15 minutes. Tried FFXV a second time with EVGA Precision X1 running this time. Precision X1 showed the GPU temp climbing but hadn't exceeded 80 degrees before the next crash. A few minutes after taking flight, I hit alt-tab and endured another hard shut down. Opened the tower both times and felt the 'heat coil' (I think that's what the large metal section is called.) It was hot enough that it felt like it could have mildly burned my hand (or at least would have started to hurt a lot) if I kept it there for more than a couple seconds.
Due to my past prolonged experience with an EVGA 970 hard crashing regularly (full shut down) years ago, these 2 hard crashes were enough for me to feel that there was a hardware issue and I decided not to spend more hours proving it. Looking back, I wish I'd done a couple more tests with my 3080 / CX750M to be closer to 100% sure. Called EVGA a week ago and their automation said to leave a message since there was more than a 30 minute wait. Left a message explaining the crashes and never heard back. First tried to update my mobo Bios but the MSI Z97 Gaming 5 drivers appear to be messed up so none of the 3 most recent updates worked. GeForce experience drivers were up to date the whole time.
Fortunately, I picked up on clues from other posts (such as this one) that I might just need a new PSU.
 
Worked flawlessly: Corsair RM850X with EVGA RTX 3080 FTW3 Ultra.
This upgrade was relatively easy because it allowed me to leave all PCIe and Sata cables plugged into the devices as before. Be careful and verify online before you try swapping cables like I did. Corsairs website confirms that PCIe and Sata cables are interchangeable between these 2 PSUs, but in other cases, you could destroy your hardware by using the wrong cables.
 
Speculation: The Corsair CX750M only has 2 of the 8-pin PCIe connectors, but each cable splits into 2 for a total of 4 PCIe connections. I recall some people questioning whether you need 3 separate PCIe connections from PSU to GPU on the 3080 FTW3 Ultra since it has 3 8-pin PCIe spots (3 times what my old GTX 1080 needed!) 
My new PSU is only 100 Watts more. I'm suspecting it may have more to do with the 3 separate PCIe connectors than the wattage in this case because I've read posts speculating this card has very high power spikes. Logically, if those power spikes get distributed to 3 separate inputs on the PSU (instead of just 2), I imagine there would be far less chance of a shutdown. 
 
Best wishes to everyone looking for solutions and thanks to all for contributing here to help others:)

aardvark1134
Make sure your UPS can handle your power draw.