Hmm... Might be mipmap quality. I had some similar issues with lines on my UI.
How to adjust:
ATI cards: Control Panel > Display > Settings tab > Advanced > 3D tab > Direct3D > Custom. Set Mipmap Detail Level and Texture Preference to High Quality.
nVidia: Control Panel > Display > Settings tab > Advanced > ... don't know from here, don't have an nVidia card at the moment.
But it'll be in there somewhere.
That's all I can think of to try. If it fixes the issue but kills your framerate, play around with settings both in and out of game a bit until you find a good middle ground.