Try cutting your thread always at the spool and pulling the thread through the needle. Do this a couple of times in case there is a little piece of lint in there. I was told never to pull thread backwards through the tension disks. Could build up lint. That said, I agree it's an upper thread tension problem. Also try a different needle. Clean everywhere, especially in bobbin and feed dog area. Could be an odd little piece of lint somewhere. Let us know if none of this helps.