Do you have anti-aliasing forced in your graphics drivers, or anything like that?
Try browsing to the ..\SteamApps\common\Divinity - Original Sin\Shipping folder, right click on the EoCApp.exe, select properties, switch to the Compatibility tab and disable desktop composition (and maybe set it to run as administrator). That can sometimes help with graphical glitches in other games.
Email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool.
