Have you seen the cloth room tutorials at PoserFashion.net? There are instructions on how to use dynamic clothing with the free dynamic dresses in their Poser 5 section. Most dynamic clothing is set up so that it "fits" if your figure is zeroed. If you start with a figure that's not in the zeroed pose, or one that has been morphed from the default, the clothing may intersect the figure (even a tiny bit, where you can't see it). That may cause the problem you describe.