I don't have a Bernina and don't have any quick answers, but if you suspect the Minky is the problem, then try a more conventional quilt sandwich and see if the stitch length improves.
A few times I have had a similar problem, and the cause was that the walking foot was not receiving any power from the needle bar. In my case, the walking foot's forked arm that transfers power from the needle bar to the walking foot was not engaged. In the case of my old Singer, the forked arm was not above and under the needle screw. I imagine that setup is different with the Bernina.
Dayle