I've playing through this again on my pc cause I got it for free with my video card, and it's been running perfectly fine. No problems whatsoever until I hit chapter 8, or actually right before chapter 8 when you first go to your apartment at the end of the chapter. The fps drops really badly making the game unplayable. As I said before I haven't had any fps problems at all with the game before this. I tried turning down all the settings and it still gave me terrible fps. I'm running it with a GTX 550 Ti with amd 2.2 quad core and 4 gbs of ram so my system shouldn't be the problem here. Is this a common problem?

I doubt there's very many people who remember Mafia II well enough to help you out, but good luck. For what it's worth I don't remember having problems with it on the 360, but I don't remember much at all of that game.