Don't know if this would apply to your setup or not, but I've found that sometimes when the thread gets near thhe base of the spool if I get going too fast it will somehow wind itself around the spool pin when I stop (I think that's what it's doing anyway) and then when I start up again it will only go a short distance before the top thread breaks. Took me awhile to reach this conclusion, but now I watch really close if I know I'm getting to that area of the thread on the spool and try to catch it before it becomes a prooblem.