My old world crashes after 90 seconds or less of playtime, and cannot be saved after that point.  If I play it will crash after 90 seconds. If I sleep, it will crash upon waking up. If I try to save after doing something or moving to another location, it will crash. If I try to take down signs to remove entities, and then save, it will crash. Size of the world is ~356 MB. Started new world that does not experience this issue.