could be: +vertex colors +flipped normals +split edges/verts/polys that share the same space +material IDs +smoothing groups +zero face polygons Try dropping an "STL check" modifier. Its sort of an old school way of hunting down broken stuff on your model.
this is similar to my problem that I have been trying to troubleshoot for the last few days. I've posted some images on the Marmoset thread. http://www.polycount.com/forum/showthread.php?t=74848&page=31 But my issues differs in that I can't find any unwelded verts in the UV map. and if you scroll down on that post, you'll…