Just a shot in the dark here. I don't really suffer lag much except in SubT. I mean it's all the time and constant. Every six seconds the game pauses for about 1/2 a second, constantly.
Once I get in an actual quest like Hound or VOD I'm fine again. I have a decent enough system i7, 6GB RAM, Geforce 275GTX with latest drivers so I don't know what's causing this.