I remember having a lot of problems with the game stuttering right after shooting the officer in the woods, and at the time it distracted me and slowed down so much that I gave up on trying to play it on my PC. I just spent enough time in the game right before this post to get through that part again, and it had the same problem but to a much lesser extent this time.
The only change on my PC in the intervening time that I can think of is having gone from Win7 to Win8. In any event, I'll probably try to find the time tonight after work to give the PC edition some more time. The Xbox version is on a Christmas wishlist, and I'm perfectly content with owning two different versions of this game even if the PC one seems to be working better now.