Your stitches look very small. I'd suggest adjusting the tension and reducing the number of stitches per inch. The pleat edges aren't taking any significant strain, so you don't need to pack the stitches too tighty.

Also, you'll save time by not trying to mark the edges and following the marks. When sewing pleat edges I keep the edge of the pleat even with the edge of the machine's foot. That spaces the stitch line just the right distance from the edge of the pleat. If your foot is too wide, use a sharpie and mark the right spot on the foot. With practice you can zoom right along, and the stitch lines will be straighter if you speed up. This will save you loads of time and produce a better-looking job.