Judging from the screenshot, you are using Poser 5, so ignore any advice regarding AO, IBL etc P5 doesn't support those... If you're using an earlier version of Poser than P5, then I can't help, so ignore the rest of this post.

The problem looks to me like the dress is made using "2-sided" polygons. P5 has a particular problem with those..

A way to check is to switch to the P4 renderer. If you then notice thin, horizontal black lines appearing (instead of the blotches) then that's exactly the problem.

Alas, there's no way that I know of to cure the problem within P5. (Not to say that there isn't, just that if there is, I don't know it).

The only way I ever found to get round it was a laborious process of:
Export the skirt as a .obj file.
Load the .obj into a 3D modeller.
Select the whole thing and convert to "1-sided".
Save result.
Import saved result into Poser.

You will unfortunately, have lost any boning (if it's a conforming item), but you should be OK if it's a dynamic cloth...

Hope that helps,
Cheers,
Diolma