I experienced this same issue when I first installed my Trio X 4090 and water block for my custom loop a few days ago. Would randomly black screen 4 or 5 times throughout the entire day, but the keyboard etc would still be responsive. I initially thought the GPU wasn't seated correctly, but it was fine. Thought perhaps the card was faulty. I just used the standard PCI-E splitter that came supplied with the GPU.
I then DDU'd in Safe Mode, fresh installed the newest driver, updated my motherboard BIOS and chipset drivers to the latest ones, and thankfully haven't experienced it since.