I previously had an issue with (presumably) textures loading, which would cause the FPS to periodically drop dramatically. Standing in the countryside, with nothing, nor no one around and merely turning my head to look to the side would cause massive hard drive activity and the FPS would drop dramatically. I have since loaded the game onto a SSD and I no longer experience that problem.
The game was originally on a Western Digital Black 7500RPM drive. It is now on a Samsung 840 EVO SSD.