Problem shooting took a new turn the last couple of posts. If it's issues with needle and thread, it's all about matching correct weight thread, needle and fabric. I'm sure you know it better than me, but as Maria mentions free motion in general needs a reasonably large size needle and sharp poit can be an advantage.
Another is cleaning, if there's lots of dust around feed dogs, hook, race and bobbin case, it can affect the stitching too.