1. Give specs and quality settings you're using
CPU: AMD Phenom II X4 955 (@3.2Ghz)
GPU: AMD Radeon HD 7970 (with 3GB GDDR5). Driver AMD Catalyst 14.9 (clean installed, factory settings except for AA method set to "Adaptive Mutlisampling" (default was "Multisampling").
RAM: 14 GB
OS: Windows 7 Ultimate x64 SP1, all updates applied.
HDD where installed: OCZ Vertex 4 SSD
Started with: Visual Quality: Awesome, Fullscreen: on, Res: 1920x1080, Antialiasing: Medium, SSAO: Low, Noise/grain: Off, Motion Blur: 0%. (the lag was too much for me)
Settled with: Visual Quality: High, Fullscreen: on, Res: 1920x1080, Antialiasing: High, SSAO: Off, Noise/grain: Off, Motion Blur: 0%. (somewhat better).
Also tried: Visual Quality: High, Fullscreen: on, Res: 1360x768, Antialiasing: Medium, SSAO: Off, Noise/grain: Off, Motion Blur: 0%. (again better, but poorer visuals).
Note that even with High Antialiasing, there are noticeable jaggy edges for the models.
2. Say what kind of framerate range you mean when you say choppy
Framerate dips below 30 fps, sometimes lower than 20 fps in the first opening scene (at parts not constantly) and sometimes when moving the mouse when navigating Zoe. I haven't reach to Europolis, I am quite early in the game.
It seems better with the lower settings, but still happens.
3. Give an exact location where you're experiencing this (careful of spoilers guys!)
First opening scene, especially towards the end of that second part (the part within some house). Frame rate there seems to be the lowest (20 fps or even a bit lower).
Sometimes when navigating Zoe (first location (more noticeable), and all the rest until the darkness person location, where I stopped playing for now).
4. And most importantly, try restart the game and see if the same thing still persists.
Btw, I am using Fraps (a bought version) to check frame rate. Is there a more reliable in-game option?