As you have such a powerful cpu and especially if the game worked OK before, I don't think it's the game itself that's causing slowdown. And therefore tweaking the game probably is not going to solve it (but of course tweaks mentioned in this thread might improve the performance beyond the original speed once you find the cure for these slowdowns).
Try to remember if you have installed any plugins after the game worked OK, and also you can test running the game without plugins (you might run into problems with loading a saved game without all the plugins that were used b4 so start a new game for testing purposes).
Also, close any programs/processes that run on the background while playing the game, with an exception of FPS optimizer, of course

. Do this with task manager (shortcut: Ctrl+Shift+Esc work on WinXP) or by closing programs from their icons in task bar.
And check if you have any malicious ad/spyware on your computer, recommend using this program:
Spy-Bot S&D