Astraweb is having terrible completion lately. As a result I am often having large sets with too many incompletes for the PAR2 set to repair.
Sometimes if I retry at a later time, some of those missing articles will have populated. Unfortunately with most large sets, this causes Newsbin to lock up with constant, heavy disk activity and continuously growing memory usage (the constant thrashing may be the result of pagefile swapping). The only solution is to kill it (which sometimes works) or to force shutdown the PC.