It's difficult to tell from your screencap because of jpg compression, but it looks like your tile is 34x36, which is not a power of 2. Correct me if I'm wrong.
Try using 32x32 tiles... or as mentioned so many times that it has now become a rather tedious mantra, "try updating your graphics card drivers."
Note that this problem is pretty much just for the Tiled Background object, because it has repeating textures (and even then, it only happens on some graphics cards). If you need non-standard sized tiles you could probably use instances of Sprites, some people have had success with that method.