I have the Janome 9000, old model now. Are you new to free motion? If so it could be that you are whipping around the curves too quickly, which creates 'the eyelash' effect on the back. I was so frustrated when this was happening to me, until a more experience sewer explained that this was in fact one of the 'lessons' she taught and I was a natural LOL Since I slowed down when going around a curve.. (think big head little neck, big head little neck...when meandering!) My tension issues improved amazingly!
I have also found it SO much better slowing down my machine speed to half speed which gives you more time to plan ahead so you are not panicking when you come close to another row of stitching. My stitch length is becoming more even too.
Of course it may not be this at all but something to consider ;)