I would check your bobbin holder parts, I get this if it is not properly cleaned out of odd pieces of threads. Also check your threading of machine and most importantly, are you using the same thread in your bobbin as your top thread, and putting a new needle in doesn't hurt! I never mess about with my tension unless by mistake I have moved it from the norm, so check all of the above and if all else fails test your tensions on a sample quilt piece.
My quilting teacher always said TNT, Thread; Needle; Tension will solve all quilting problems and she has been proven right 100s of times :-P