I'm running Win8 64bit with a computer that, given the game's age, should be easily running it and indeed the game ran
perfectly smoothly till I reached the Nosferatu lair.

After that the game drops to around 10-15 FPS seemingly at random when I talk to someone, travel between areas or
view my inventory.
It does not do this every time I perform one of those things, it just suddenly seems to decide that now's a good time to drop the framerate.
It even lingers on when I go to the game's menu by pressing ESC, but if I exit to the main menu from there, the FPS pops back to normal yet instantly drops down again if I load my game.

I've tinkered with the settings, having the game at minimum graphics & resolution, running it in various compatibility modes, running the game as non-admin (I normally run it as admin) but the issue isn't going anywhere.

The only way to get rid of it temporarily is to close and restart the game which allows me to play normally for a while till the problem occurs again, requiring another restart.

As that results in a rather poor gaming experience, I'd appreciate any advice on the matter.

1. Reinstall and see if it helps.
2. Try an earlier savegame if you say it has been ok until Nosferatu lair.
3. Override ingame vsync (for example: set Adaptive V-Sync for the game through the Nvidia Control Panel).

That's all I can think of.

I attempted a reinstall, but that didn't appear to change anything.

I then attempted to start a new game entirely which is where things got.. weird.
After creating my character I'm instantly dropped to the alley you meet Jack at, except there's no sign of him or anyone else.
There's the Pestilence (or what's it called?) effect from the Sheriff's spell he does in the cutscene in one corner, it does nothing if walked into.
All doors save one are untargetable, the one targetable door is locked and unpickable.
From everyone around I can hear constant gunfire and terrified screams

That's where I now am dropped at every time I try to create a new character.

So err, how would one go about with option number 3?

You should start by telling us a little bit about your computer and operating system, then tell us if you are using any mods or patches.

It sounds like your game installation is really screwed up. How did you uninstall it before reinstalling it? Same location? Did you let the installer verify itself during installation?

Win 8, 64bit.
Intel(R) Core i5-4440 CPU @ 3.10GHz (4 CPUs), ~3,1GHz
8192MB RAM
AMD Radeon HD 8470

No mods outside of the Unofficial Patch that comes as default on GOG.

Uninstalled the game via Galaxy's uninstall option, reinstalled the same way to the same location (far as I saw the location only contained my earlier savegames).
Ran the Galaxy verification after trying my saved games and concluding that something's seriously wrong with starting a new game - it did seem to do something as it downloaded for a while, but nothing actually changed in the game.

I used the installer and that allowed a new game to be started proper, so I ran the game to the Nosferatu lair and amazingly, the FPS issue didn't occur now.
If I imported my Galaxy saves though, they did show up but only for that character.

Regardless, thanks for the solution!