If the bad stitching is on the top, the problem is with your bobbin. If the problem is on the bottom, the problem is with the top thread. Either way, rethread very carefully, and clean out the bobbin case well and rethread that carefully and see if those help. Also, you might try changing thread spools if the problem is on the top and you are getting down toward the end of the spool. If the problem is the bobbin thread, check and make sure the bobbin thread still looks evenly wound. If it is towards the end of the bobbin, I would go ahead and put a new bobbin in. Good luck, and let us know what worked.