Yes, I believe you are correct it is the tension. I would take a scrap of the fabric you are sewing on and start by adjusting the tension a bit at a time...do a straight line, stop and remove the scrap, take alook at the fabric both sides, replace and adjust up or down as needed until you hit the perfect stitch....check for puckers and pulling. It might take a few tries and just keep at it. You will find it. I tend to adjust my tension dials by l number (mine has numbers around the dial)....if it is too loose on 4 and too tight on 5 then I turn it between the two in that gray 4 1/2 area. I hope this makes sense. It might also be an incorrectly wound bobbin...check your manual. I hope you get it worked out. Budster