Thank you for that information. I did play with the antialiasing setting, but except for a tremendous increase in rendering time I did not achieve any positive effect. The dots were still there.
http://sdrv.ms/14Wl3IS . obviously it would be great to get this solved.

On another rendering issue: I figured that textures sometimes don't render properly. just compare the TV on the wall http://sdrv.ms/14Wl3IS with
http://sdrv.ms/14LYR36 . the only difference is the viewpoint. It is NOT depending on rendering mode 3 or 4, and I have not yet figure out in which cases the texture is hidden and when it works. I did create the TV with sketchup (from scratch) and position a texture myself there and exported it to .obj.
for completeness: In rendering 1 and 2 the texture works always.