I don't have a Bernina but I have a Babylock that has somewhat the same problem. At first I thought it was because the bobbin winder did kind of an odd job on the winding but then later I put in a prewound bobbin and it did the same thing, though not as bad. It's in the shop now. I don't fix them, I just ride them hard and put them away wet! If they don't sew to suit me they go to the shop. (This is the third one since November!_