After lots of testing, I figured out the problem: it's the screen resolution.
The crashes that restart the computer aren't because the GPU or CPU is too hot, or because of mods, or bad/faulty hardware. I even ran the game on a GTX 650 for a whole day with no crashes (it was super slow, but stable) — and it doesn't matter if the computer is online or offline.
The problem is this — the higher the resolution, the more unstable the game becomes over time:
1080p - No restarts, super stable even after 100+ hours.
1440p - Some restarts start happening after 20+ hours, mostly during cutscenes. For example, the Shadowheart romance scene when the tieflings visit camp always crashes.
4K - After ~12 hours of play it's a constant downpour of crashes. Almost every cutscene, quicksave, location change, inventory access causes full system restart
Solution: don't buy 4K or 8K screens for gaming — it-s nothing but trouble and a waste of money on hardware. Lower the resolution to FullHD (and enjoy the blurry, soapy image)
