Can you try to post a render of the mesh with a neutral light grey and zoom in on an area where it occurs. Also if you can just screenshot the preview window at the close up. 

To me it doesnt look like it have anything to do with the IDL but the mesh it self, due to Posers way or way not to handle smoothing very well :)

 

To test whether its that, try the following with the roof part, as its probably easiest to see there. Select each of the polygons facing the same direction, meaning all those around the edge where the lighting goes wrong and then detach it to its own element. Personally i use 3DS max so if you use something else i wont be able to help how to do it. But once you have done that, try to import it into poser again and see if it still happens.

 

Whether thats the problem im not sure, but to me at least that seems to be it.