PART 2 – (Simplicity 1198) Thinking of leaving off slee Sewing Machines Parts and Sales in Los Angeles

Hey folks, I’m back with a technical question. I decided to go ahead and finish the shirt as is with the sleeveless/cap sleeve look. But I’m running into a sewing noob problem, and I don’t know what to call it so google isn’t helping me. I used a friend’s serger to serge the edges of the knit to keep it from unravelling, then I decided for the armholes to just fold it under once and do a stretch stitch. I cut some of the little wedges in the edge so that it would be able to curve, but I’m still getting this weird ripple effect after a few inches of sewing. What am I doing wrong?

Picture of weird ripples
Original post for reference