Hi Isyl, maybe--check how many parts you have to the hair (Materials Room-especially the transparency settings).  Click on the part of the hair that is okay-record the settings and nodes, go to the bad part and see if there are differences (a node not connected, transparency setting not the same etc.

 If everything looks okay, then to eliminate the hair as the source of your problem, put on another hair piece-if everything clears up, its definitely a portion of the hair object that may have become corrupted.  If you have multiple parts to the hair, make portions invisible, if the problem goes away, its in the part that you made invisible, narrow it down and you will find the culprit.

I had a problem with a hair piece (bang portion).  Whenever I tried to render the scene, Poser would simply close leaving me with my default windows' screen.  As soon as I made the bangs invisible, my problem went away and everything rendered fine.  Jan