Hi Rachel, sounds to me like you're doing eveything right and from your stitches where there are no nests, I can't imagine it's your ineptitude at FMQ as they are fabulous! I FMQ with King Tut in the top and BL in the bobbin with no issues (I have a Janome 6600) so I don't agree that it can be because one is cotton and one poly or that they are 'unbalanced' - I nearly always use BL in the bobbin and cotton in the top and BL behaves with everything, even heavy decorative rayons. If it was a tension issue, I think your stitches wouldn't be so good between nests, it would be consistently off. A machine of this quality should not need any bobbin washer gizmos - those are intended for machines that aren't really calibrated for FMQ. I assume you're using proper Bernina bobbins and not universal ones? So I think it's either a fault with the machine or a burr in the bobbin case or something stuck in there somewhere. I would say give it all a good clean (I know you said you have but I'd do it again!). Wind a new bobbin full of BL, put in a new needle and rethread the top. In other words, eliminate eveything possible from the operator error category. And then go to work on a practice sandwich of the same fabrics and batting as your quilt. If it STILL does it, then it needs a Bernina registered dealer. I hope that is helpful, I FEEL your pain. :-( :D