I second the idea of checking the bobbin case carefully for stray fibers. Also double-check you're threaded properly.
On my Janome, I don't have to adjust tension for FMQ. If my stitches start to go bad it means either something is stuck in the bobbin, somehow it's come partially un-threaded (sometimes my thread manages to fly out of the pick-up arm during FMQ), or I'm just moving too fast.