I know what you mean. I did the same thing yesterday and kept having to up the number of max items, then looked at the stats and it was always above, so kept upping it until eventually it worked. I suppose you may be having the issue because the size of your model is so large. It might be worth trying what the others are suggesting and use a texture, or cut down the area to only what's ever seen by the camera.
Sorry can't be more helpful!