Hard to tell without knowing what exactly you are doing in that event/network parts and if it needs synchronization, but basically, you shouldn't modify jPCT related objects like (the World, all Object3Ds...) from another thread then the rendering thread or you have to put a synchronized-block around these parts as well as around the rendering part and synchronize this with some generic sync-object (or whatever is appropriate in that situation). You may want to have a look at the advanced example in the download section. It has some client/server-code that does this. However, albeit its documented, it may take a time to get a grasp on it.