Sanek, could you possibly upload one or more of your save files in which you are experiencing the framerate issues? According to
this post, nVidia are not able to see the issue on their end, thus believe there is nothing to fix.
If you are a Windows user, and using Steam to run your game, you should be able to find your saves in the below folder. Since you claim the FPS drops a lot at Lambda, perhaps your save file(s) from there would be a good start.
C:\Users\<user>\Documents\My Games\Soma\Main\<steam_folder>
If you use something else, I am unaware of where saves are stored.