I have experienced the same types of hang-ups in Poser 5, but have come to the conclusion that it is not the light sets per se that are casuing the problem because it doesn't happen consistently all the time. Rather, it seems to be more of a universal problem that P5 has when you make several CHANGES to a scene whether it is swapping textures via MAT pose files, loading different light sets, or applying materials in the material room. If you do it several times in a row, Poser just freezes. I think it has something to do with P5 not properly clearing the previous element before applying the new element resulting in some memory allocation error. The result is a "build-up" memory leak that eventually overwrites the poser core. To test the light set try doing a fresh reboot, start Poser and apply the light set before making any other changes. If it renders properly under that circumstance then it is not the light set. If it crashes Poser, then it is probably due to bad formatting of the light pose file itself. Improperly formated text files (whether pose files light files cr2s, etc, are known to cause problems with P5.