One thing that helps me when I'm sewing long strips is to just plain slow down. I know it takes longer to get from A to B but it seems to help the strips keep straighter. The other thing I'm wondering, when you cut your strips and opened them out, were they straight? Or did they have a little wow in them already? Sometimes cutting on a fold that is just a little off will throw out your strips as well. As far as stitch length goes, why would you want to lengthen the stitch? For sure as others have mentioned, when you slice your strip strata if the stitching is too loose, you'll have seam ends unravelling. Don't lose hope, we've all been there and that's why we're able to come up with so many maybes :roll: