breakage- and nesting usually means---a tension problem---or more likely- some lint built up in the bobbin area-or the need to change the needle-
it is recommended to change your needle for every project-or every 8 hours of sewing time-which ever comes first- and the number 1 reason for poor stitch quality is a dull needle-
that's not to say it isn't your thread- there have been times when i have had to change my thread 3 or 4 times before i finally found one that was working for my project- but making sure the machine is clean (the bobbin area should be cleaned of lint every 2nd bobbin change-if not more often-some fabrics create more lint build up than others-
a new needle---the correct needle size/type for the thread being used (if you do not know-visit Superior threads-they have free charts you can print off telling you what needles to use with what threads for what projects)
make sure the machine is threaded correctly (it is easy to miss a step-even when you've been sewing for years)
make sure the foot is lowered- we all forget that once in a while (causes messes/nests on the back)
if all else fails-then try a new spool of thread- everyone has their favorites- mine are Connecting Threads thread- but i also use threads from Superior Threads, Atlanta Thread Company, YLI, Sulky, Gueterman, ...the important part for success is the correct wt thread for the correct size/type needle- and a new needle, clean-well maintained machine.