When you get noticeable artifacts (like the one in your image), select the Body of the particle figure and try rotating it slightly on the X or Y axis (there shouldn't be any significant change to the shape of the cloud). At times, Poser may need this bit of "convincing" to make the angles work with one another. The clouds are more susceptible to line artifacts because they're relatively opaque and rely mainly on diffuse color. Sprite-based systems that are more transparent and rely mainly on ambient color are less likely to give you line artifacts (sparks or fire would be a good example of these). Hope this helps!