Editor keeps freezing during build

Hi, all, I'm using u4.5.5f1 pro on Win7 and recently I've been encountering weird build issues here. I have around 30 scenes in my build, and when I start building, it usually freezes at building some level( the progress bar no longer advances, and Task manager shows no activity on Unity process for long time). This problem shows up frequently, and sometimes restarting PC can finish one build. I checked with the editor log files, and these are two errors log in there, but they're not coming from my scripts. **m_ThreadedObjectActivationQueue.empty() && m_ThreadedObjectActivationMap.empty()** **ms_IDToPointer->find (obj->GetInstanceID ()) == ms_IDToPointer->end ()** Any help is appreciated,