You mentioned ALT+ENTER so I assume you're using D3D11 or 12? Generally the recommendation is to start with the game's default settings which are
Direct3D 8
No Anti-aliasing
No Anisotropic Filtering
All shaders off
And try that first. To speed up testing, make a sandbox account, create a character, make an Episode 4 game, and you can test quickly by doing /warpme 9 to change to the Episode 4 boss map (assuming this is the cutscene you're talking about at the boss). Then slowly try the settings you were using until you find the culprit. The game originally had only only those settings. The other renderers, shaders, AA, AF settings were all added here. They work great for some people and not so great for others. Unfortunately, you kind of just have to test to see what works for you.
The game screen freezing is an issue some people hit with D3D11 or 12 iirc. I don't know why, maybe the VRAM setting isn't sufficient for the other settings.