Created simple terrain with textures, but getting these white lines?

I’ve just been making this landscape with some textures, but when i preview it these white lines show up - Imgur: The magic of the Internet . The white line actually move/recede as i walk toward them (are they the segments of the map?) Strange thing is that I’ve tried previewing on a different computer (with a better GPU) and these lines don’t appear. Any idea what they could be?