I can almost guarantee that the problem is a bit of thread caught in the bobbin area. Just because you don't see it easily doesn't mean it's not there. David McCallum's book will give you detailed instructions on how to fix the problem. Tip: get a strong magnetic dish at the auto supply store to hold those tiny screws, and magnetize your screwdriver while you're at it. Also, make sure you have no distractions (kids, pets, etc) when you are removing that tiny gib screw. Dropping it will cause you to increase your vocabulary..............!