I think if codecs were the problem, the stuttering would be in the cutscene videos, rather than during the game itself. Some people without a codec pack have installed one to fix problems, while others have uninstalled one to do the same, so it is system dependant.
It is possible something installed, updated or changed in the last couple months could be causing the stuttering...
You can try emailing support@larian.com, with a description of the problem and maybe a link to this topic. Include the report from the
support tool, and maybe a dxdiag report (In Win 7 click Start, then enter "dxdiag" into the start menu box; In XP, click Start, then Run, and enter "dxdiag" in the Run dialog box. The DirectX Diagnostic Tool should then appear. Click on "Save all information" and save the file to your desktop, or somewhere handy to attach to an email).
