I've had this game a long time and played it on many systems. I can tell you it's probably just the game. I guess it's not optimized for more than dual core.

Sometimes it runs great. Other times the frames will kind of skip every few seconds or so while running. Even while staying at 60fps. I've been kind of able to minimize the slight micro stuttering by forcing vsync and tripple buffering in Nvidia control panel (game profile) and also set max frames to render ahead from default which is 3, down to 1. I also disable threaded optimization and set power management mode to prefer maximum performance. It still does it from time to time but these settings seem to help for me at least.

If you are losing frames in combat you may want to experiment by lowering some in game options. Maybe set anti aliasing from 4 to 2 or off and anisotropic from 16 to 8, try shadows on medium. If that makes no difference then it could just be another problem of the cpu optimization. Also just by enabling vysnc like I suggested above may help with this if you haven't already because it tends to just make things smoother overall.

As far as the Trade Quarter goes in Act 3, I'm pretty sure everybody looses frames there. The best thing to do is either deal with it (since that's about the only area like that) or just turn down view distance from max to medium for that area then put it back up after. If you don't mind the way it looks with the far off view being foggy you can just keep it that way, it may also help with your lag during combat.

It's a really good game despite it's little bugs. Like sometimes running really fast for no apparent reason.