[2021] On full screen and window screen (medium and high), screen goes black after a bit.

Has this always happened ever since you've had this game? Or only since when?

When you say your screen goes black, do you mean the monitor turns off? Or the computer hangs or powers off? Or what exactly? What do you have to do to recover?

DXDIAG is a program you run from the Windows search box on the task bar. After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.

Please attach here your Documents/Paradox Interactive/CK3/pdx_settings.txt .
From your Documents/Paradox Interactive/CK3/logs/ folder, attach here system.log , error.log


Please report the temperatures of your CPU and video (GPU)
1/ before starting the game up and
2/ after say 10 minutes' play.
If your PC's vendor has given you no way of reading temperatures, install
SPEEDFAN http://www.almico.com/sfdownload.php  

AdventOtaku said: when I did play the game at max, it ranges from 80 to 90ish degrees.

That is the cause of your problem then! I will guess that's your GPU temp, not CPU, would that be right?

You are running in windowed mode and vSync on, so the game will be at 144hz , the same as your Windows desktop. If you put it in fullScreen mode, disable Vsync, and cap the fps at 60 does that temp reduce to be below 80c the whole time?

If it is still too hot when run like that you likely have a hardware problem. Clean out all air vents it has (compressed air cans are good for this). Make sure airflow to them is not blocked by anything like walls, books or whatever.
If that doesn't drop the temps enough, you probably have a faulty fan or a heatsink/thermal paste problem. You may need professional help at that point.  

Anything over 80c is still a problem IMO! I'd suggest you look into your cooling system as I described above.