Usually problems started on 1st level. What happened?
Gneral solutions: game as any games should work on native for that game FPS. Stable, not lowest, not highest. Nowadays Vsync better to be off, originally it needed to limit FPS. Now it can be replaced with some different thing (Gsync). Game should work\forced to FPS is 60.
Another solution - switch resolution to 800x600 and keep aspect ratio in video driver. Should be black bars.
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Using GOG Galaxy same as for Steam require game files verify after downloading. At least twice. Or reinstall via offline installer.