Hi!
I know yours was an older post, but I have had the same problems with my 440. It shows up the most with black thread on light fabric. The stitch is crooked, and every once in a while the bobbin thread starts looping. The dealer has said to "let the machine rest", "that's just the way it sews", etc. I wrote to Bernina and they said to bring it to the dealer. I'll have to spend some time there with them. I had a 150 for about 7 years before that, loved it and had no problems. Glad you got a machine you like. Thanks for posting your problem!!