Besides all of the really good suggestions already posted, make sure bobbin area is clean, and check under the plate also. Lint can really bunch up in there and is hard to see. I've taken my feed dogs out as I only use this machine on a frame and it makes it easier to clean.
Sometimes we tension the quilt too much, you don't want to bounce a quarter on the quilt. Should be able to grab your finger on top if you poke it up under the quilt layers.
Also is the thread fraying? If fraying and breaking you probably need a larger eyes needle. I use Schmetz topstitch 16-18, or inspira topstitch needles (they are great).
Check to see that the thread isn't catching on something along the way, and that it's threaded properly. I use pre-wound bobbins with King Tut on top, and the odd time a poly on top with good luck. Have only had trouble with threads breaking on one quilt, tried everything I could think of, switched out the Sulky thread I was trying on top, and she ran like a charm.
Wishing you luck.
Laurie