Argh, I saw a tutorial on how to fix this somewhere... Basically, you need to get a small file and file out those nicks that are causing the thread to shred. I think there might be a sanding cord of sorts you could use? I'm not sure paper alone would do the trick; you'd be more likely to get straight ridges than a smooth surface, I think. I'll try to find the tutorial, I swear I've seen someone talk about how to fix this.