the grid is a bug.
I have this with my ati graphic card. Because, all tile are not multiple of tow.
i use always 64*64 or 32*32 for tiles, never 70*70 ^^
look at this
<img src="http://puu.sh/5TGvr/27269e8d91.png" border="0" />
no grid , and i use 32*32 for all tiles