carol45

Maybe it's happening when you reach a seam on the back, so it has to slow down to get over the hump? Reducing the pressure could help with that as well.

PumpkinHead

Thank you everyone. I was doing several things wrong. Once corrected, the inconsistency went away.

Quilt drag was, indeed, the culprit. There's how I fixed it:
1) Leveling all the tables I had brought near my sewing table
2) Pooling the quilt in front of the presser foot and ensuring nothing pools behind the presser foot
3) Swapping presser foot from the open-toe applique foot to the recommended quilting foot (0A in this case)

Now I can frog the inconsistent quilting lines, fully confident in the ability to improve the results. And there is still some time left in weekend to quilt. Wahoo!