Guessing the OP is on a HDD. Only two ways to resolve this. Add more ram to your system or move the game to an SSD. What is happening is, the game streams textures from the ram and you're running out. When the ram isn't available, it turns to the Hard drive. I'm currently running the game on a hard drive and 16gb of ram and I have no texture pop ins. I've also heard of people on 8gb resolving this by switching to an ssd which makes sense. I personally would recommend going with the ram because it will benefit you not only this game, but others as well. You don't necessarily need an ssd for this game (if you have the ram) but an ssd is just nice to get in general.