I know your pain!! I just called the place (LQS...I have a Viking Topaz 30) that I got my machine on Friday with the same problem. I tried EVERYTHING!! Re-threading a million times, tearing the bobbin case apart, changing the needle etc. In talking to their tech the first thing (and always the first question) is what type of thread I had Clark & Coats....I had just finished a t-shirt quilt and it worked fine, I was sewing a pinwale cord. dress for my grandaughter and I had to "fight" with the machine. Finally got that done and had started on a cotton dust ruffle with the same problems. He had told me he could look at it but the first thing he was going to do was change the thread to Mettler. I had some so I changed the thread to the Mettler and it sewed like a charm again. He told me that C&C has a wax coating on it and sometimes causes the problem. The funny thing was that I sewed a few rows (with the Mettler) on a scrap and changed back to the C&C and it continues to sew like a charm. I have since made 3 sets of flannel jammies and finished my dust ruffle with the C&C and all is well. (at this point anyways)
So I guess try changing the thread brand and see if that helps??? Good luck.