Whenever that happened to me, it was because there was drag on the quilt. For quilting on a domestic machine, I think the "suspenders" idea is a great one for relieving all drag. Some people make their own out of PVC, but this is the commercial version:
http://www.jennoop.com/suspenders.html . There are a couple of Youtube videos that demonstrate how it eliminates drag.
I notice in your photos that the stitch seems to go small as soon as you dip over the seam to what seems to me to be the "high" end of the seam, where the seam allowances are. It may be the change in layers that is triggering the problem. I agree with trying to lengthen your stitch a little.
I am also wondering if your machine has an adjustment for the distance of the presser foot from the feed dogs. Mine does not, but many machines do. There may be an issue with the presser foot being too close to the feed dogs for the number of layers you are feeding (or perhaps the opposite?).