I would also suggest changing or checking the needle. Make sure that the flat side is to the left and inserted as high as it will go. Even if the needle looks ok it may have a burr or be a little blunted and this will affect the quality of your stitches. Frequently poor stitches are caused by faulty needles and not by bad tensions or faulty threads. I'd also check to see that the bobbin is inserted correctly, the bobbin thread unwinding easily (make sure bobbin wasn't overfilled), and the bobbin case pushed all the way in. You could also have a burr or little nick in the throat plate hole and the thread is catching on the way down but it is more apt to be the needle. I hope one of these suggestions results in happy stitches.