Why does my display keep flashing after a cold boot?

For about 10 months now, I've been dealing with a frustrating issue where my display starts flashing after a cold boot—especially if the PC has been off for over 20 minutes. Initially, it stabilizes after 2 to 4 hours of using my PC without opening any programs, which isn't ideal. This problem even occurs on the BIOS screen before Windows starts. Here's what I've tried so far:

- Replaced my GPU (I'm currently using a GeForce RTX 3080).
- Swapped out the PSU twice (now using an Antec Atom 750W).
- Changed the CMOS battery.
- Installed a new motherboard (ROG Strix B550-A gaming).
- Reinstalled my graphics drivers (although the issue occurs even in BIOS, so I doubt that's the cause).

I've tested both sticks of RAM individually in both motherboards, but there's no change. They're 2 x T-FORCE 16GB DDR4 DRAM 3600MHz. Interestingly, while using both sticks in the right slots on the new motherboard doesn't work, each stick works fine when tested alone. I suspect a seating issue on my part since I reseated them several times.

Also, I had another issue where the PC stopped displaying in the middle of use about 4 times over six months, but that hasn't happened since September. I'm feeling quite frustrated since the most recent cold boot problem didn't stabilize even after 8 hours of use. Now, I'm leaning towards the CPU or RAM being the culprits, but I really don't want to spend money on something that's not the issue. I desperately need this PC for work!

Oh, and I almost forgot—I have tested using another monitor and different HDMI cables, which was one of the first things I checked.