Framerate issues with Amnesia: The Dark Descent

[AdamAK]

Summer hasn't really arrived in my country, sadly. Besides, this issue started in early March (quite suddenly), when it was still pretty chilly out here. My idle CPU temperatures are sitting at a regular level.
Furthermore, my other games run fine on this laptop and only Amnesia is giving me trouble.

I'll clean my fans again, but I'm quite sure the problem I'm having is not really hardware-based.