There are a couple of things mentioned in this thread that I would like to expand upon.
Regarding thread and needle size: What kind/size thread are you using? You mentioned that you switched from a size 12 to a size 14 needle. I found that when I used a 40 wt thread (King Tut, which is fairly thick), I needed to go up to a size 16 topstitch needle. Topstitch needles have a larger eye and "scarf" or groove in the side of the needle; this protects the thread from a lot of friction as the needle moves in and out of the quilt sandwich. A 16 will also help with any skipping issues because it's thicker and less bendy. (I know you didn't say anything about skipped stitches, I'm just throwing that in for future reference.)
Regarding thread position: Watch carefully to see how the thread comes off the spool or cone - is it sideways? Does the thread twist off the spool straight up? Or does the spool spin? Sometimes the way the thread comes off the spool makes it very susceptible to twisting and knotting, then fraying or breaking as it moves through the mechanisms of the machine.