Whenever I've had problems with my thread breaking it's usually one of three things...
1. wrong needle for the type/size thread I'm using
2. top tension setting is too high
3. needle has gotten dull
Make sure that you put a fresh needle in your machine whenever you finishe a fairly large project
Try different types of needles and needle sizes with the type/size of thread you're using as well as the fabric/batting you're working on
Check your stitches closely to make sure there is no bobbin thread coming up to the top of your work, or top thread showing on the back which would indicate that the top or bobbin tension is too tight.