I have the same basic thing happen to me as well. These are the steps i take first . Delete RSR in Geometries folders for any models i am useing in the scene. Try again to render if still not working i move on to removeing ALL bump maps. Some reason bump map channels can become bad and need to be resaved through a paint program. its just a matter of starting some were and working through a list of things to try. It could be your hard drive is in serrious need of defragging. It could be you need to run correct reference . Try rendering with out shadows . Your pz3 could have been corupted for some reason try going to a earlyer save one if you have it. Thats my basic list of places to start at and things to try. I have split a scene into a few parts like sugested and done it that way as well.