So, I checked the master clock and it jumped around right after the SD card capacity and access time calculation with around 400 counts. But after 30 seconds, it changed around 20 counts maximum very slowly.
EDIT:
Just played another SA-1 game. OSSc loses again sync when the screen turns black and some menus are completly broken/ whole screen is just glitchfest.
EDIT 2:
Another update. I upgraded the temporary 16GB MicroSD to a 128GB MicroSD (which has much faster access time). But now MSU-1 Video drops sync too. I wonder if the SD card draws just too much power? Everything points to the power supply now. But after powering it down and up, it worked fine... I tested it with another 400 GB card and that worked fine (except the high access time). It's really hard to pin it down. ^^"
EDIT 3:
More updates:
I bought another generic power supply which delivers 2.2A, no change at all. So the power supply delivers enough power. I ran the diagnostics, everything passed. Finally, I downgraded to 1.10.2 and look at that, progress! Winter Gold on my NTSC consoles shows a complete and stable picture. It still drops it at all black screens and sometimes during gameplay or in the menu, but it's massively improved. For now, it seems like an firmware issue maybe?
Tested it with US version of Starfox and it loses sync while in the menu and it crashes after a minute.