I have the exact same issue. As soon as I enter the lower city this starts to happen. If I go back to previous areas, the issue starts happening in areas where it didn't happen before. I followed all of the fixes you tried as well and still no fix.
I uploaded a YouTube video of the issue showing from before I enter the lower city including the loading screen just to show there is nothing in between. You can skip to 2:00 to avoid the loading screen.
