Okay, so I feel silly. Decided to check one more thing. I had changed needles (the one on the machine came with the machine, I wanted a fresh needle). I put on a Schmetz Quilting Needle, size 90/14. Was told those were the same as the Universals 15 x 1H (both say 130/705, the Quilting ones say 15-Q, the Universals say 15 X 1H). My machine doesn't think they are the same! Also, when I stood them up next to each other, one of them was just a shade taller than the other (forget which one now, think the quilting one was a smidge taller).
More consistent stitch quality with the Universal 15 x 1.
Lesson learned - needle type matters - a lot.