I also use Microtex needles.

My instant reaction was try another needle. It worked for me a few days ago.
Berninas are very sensitive to any change so completely clean again , drop of oil , and re thread everything, plus new needle.
Hope try 2 works.

I too have had this problem. Sometimes it just does not like the thread you are using but more times it is the needle. Mine likes a 90 U for most quilting with my BSR. You just have to keep trying different combos until it likes your combination.

You might check your bobbin casing to see if it has a little "U" shaped piece of metal in the bottom. If not you may be getting a bit of backlash. The piece of metal stops that, and I know my Bernina does not have the piece. You can get little circles that fit in the casing before you put in the bobbin, bobbin genies. They stop the backlash and help with the stitching.

I was doing FM quilting last night onmy new Bernina. I was using the #9 foot without the BSR. I had loops on the back, ugly loops. I had to turn my top tension up quite a bit to get rid of those. I have noticed my machine is happier when I have the same thread in the top as in the bobbin. Sorry for your frustration, that is maddening!

Your needle size will depend on your thread. What thread are you using and what size needle are you using? What do you have in the bobbin

Can't help you but I had/have the same problem with my Bernina 820. I have had other commitments and have not had time to play with it some more.

Thanks to everyone for all your suggestions. I repeated all the cleaning steps and even took the cover off the thread path to dust that. I also added oil, though it was just done recently. So I am not sure what fixed the problem, but last night I was able to get all the way across the quilt with only one missed stitch ( most likely speed related!). What ever the cause, it seems to only be when using the BSR. This was also more frequently on the brown (of course it shows the most), so maybe slightly stiffer fabric. Anyway, I am much happier now. Need to get this on finished so I can start the Frozen quilts for dgds. They live overseas and will be visiting next month, so I would like to get finished so they can take them home. Again a million thanks to all the great talents on this board. Love you guys.

Get some large polka dot fabric, layer a large piece and practice meandering around the dots. Don't forget to breath and put some music on to help you with your smooth flowing motion. Looking at your sample after all other problems have been checked it probably is you are not giving the machine enough time to form the stitch also go to a larger needle size maybe a Jeans 90/14. The eye is larger so the thread can go through easier. But remember you have to breathe. Have fun and let us know how it goes for you.

Sometimes when my bobbin is not wrapped right mine will do that ,change your bobbin see if that helps