Okay, while cooking dinner I sucked in 1 MB and looked at it. I notice you have solid blacks and solid whites with no pixel light leakage at all visible in the Jigsaw and Key Caps icons. This suggests the other gridding could be a dither/shading/rendering choice on lightly colored areas, could this be so? If you experiment with the number of colors, desktop pattern and the rendering themes, does the gridding vanish for some choices? Do you see any gridding when displaying color photos that you have taken, or is it mostly in desktop area renderings?