I have a Win7 64 and my roommate XP 32. We both bought the game via Steam. I have the NPC-Problem in all its facettes. He has absolutely none of it.
So either this is related to Win7 or to 64bit OS'.
Just thought I'd share.
Yeah, I think you are on to something actually. Called my both of my friends, One has Vista 32 bit, and the other has Win 7 32 bit, and they don't have the npc stutter. So it may be a 64 bit issue indeed...unless someone with a 32 bit OS can confirm that they have the NPC issue as well.