Question Black Screen when Fullscreening Games/Discord with New GPU

[Fatal_Pastry]

I recently suffered a GPU hardware failure and had to replace my old 2080ti. I opted to replace it with a 4080 super, but I've been having significant issues with it since then. In addition to quite bad and frequent FPS drops and lag spikes, I cannot fullscreen games or discord streams anymore. If I run it windowed or borderless windowed, it is fine, and I can also see the game running in the preview widget when I alt-tab. The game renders in the background when I tab out. Sometimes tabbing out and back in fixes it, other times the screen just goes black again once I tab back into the game. I've tried all the usual basic debugging steps but have not been able to fix either of these issues, so I'm hoping someone can offer an idea I haven't tried. Here are my specs:

CPU is an Intel i7-7700k
GPU Nvidia GTX 4080 Super
850-watt Corsair PSU
32 GB Ram, fairly new
OS is installed on an SSD, also new
Mobo is an Asus Prime Z270-A

I know that motherboard is bottlenecking the GPU as it's only got PCIe 3.0 slots, but I don't think that would be causing these sorts of issues, would it?

I've tried all of the following to debug this:

Reinstall drivers via Geforce Experience
Use DDU to do a clean reinstall of the latest drivers and GFE
Checked drivers for my monitor and CPU are up-to-date as well
Tried different display ports on the GPU as well as on the monitor, and also tried a new cable and a different HTMI cable
Disabled G-Sync
If the games in question had a frame limit cap, make sure the framerate limit matched the refresh rate of my monitor
Disabled V-Sync in the games in question
Force-Rebooting my display drivers when the issue occurs using Ctrl-Shift-Windows-B

On the weekend I intend to try upgrading my PSU and if all else fails just returning the damn GPU and getting a different one. Are there any other debug steps I may be missing? I appreciate any suggestions, thank you.

 

[Fatal_Pastry]

Ok, I managed to figure out what was causing the issue. It was GeForce Experience's Instant Replay feature, specifically when I enabled desktop capture. That caused non-borderless-windowed games to show a black screen instead of rendering properly. However, I used that feature for 4 years with no issue before changing GPUs, and I make regular use of that feature for recording discord audio alongside game footage, so I'd rather like to be able to have it working. I don't know what would cause it to break this way, though. Could insufficient power for the GPU be to blame?