Forgot about the problem my friend was having with her Bernina. She worked and worked on getting her tension correct. She took it to the Bernina shop without a bobbin. They used one of their and the machine stitched correctly. Carried the machine home, put in a bobbin and the problem reappeared. Back to the shop showing them the ugly mess on the back side of the sample. This time she took her loaded bobbin along.
Conclusion: It was her thread and it was a very good cotton thread but it just would not perform in her machine. I use this same thread and have no problems with my Janome.
Suz