Funny - I had just posted in the vintage sewing machines forum about a problem with stitches on my Singer 15-91 (irregular). I had missed this post. Felt silly after discovering that my problem was the needle. Had put in a Quilting needle as opposed to a Universal 15x1. According to the lady at Joann's - both the same. I thought they were the same. My machine apparently doesn't think they are the same. And upon standing both needles upright next to each other - one of them is a smidge taller than the other. According to what I am seeing, it seems they should be the same. But they don't work the same in my machine.
Changed the needle - stitches better.
Thanks for posting this!!