The other machine that can do weird things is a slant-o-magic and others with that drop in bobbin. There is a little place where the thread passes through when it goes around the bobbin - it's a small space and there is a spring holding that. If it is maladjusted the stitches can be way off. You can tell it is off because the thread will pull through the bobbin area funny when it passes that split.if you watch and see the thread not passing around the bobbin area easy and smooth - if it goes jerky like - it needs adjustment. That will affect the stitches. People blame tension for everything.