I've just got to Novac and had been enjoying a high FPS since the start. However, as I approached Novac it started to drop. I walked to the eastern edge of Novac and looked back; my FPS increased again. Whenever I look out to the east (it looks exactly the same as any other direction; just mountains) my game starts to stutter. I've been playing on max graphics with no problems until now.

Specs:
GeForce 9800 GT (latest drivers)
Intel Core2Duo e7300 (2.67 GHz)
4 Gb RAM

I've fully patched F:NV too. Any ideas what could be causing this?

Thanks