Sewing zigzag on chiffon with Bernina Activa 230

I'm a veteran seamstress, but I am stumped by this:  I have just bought a new Bernina Activa 230.  It's wonderul after my 30-year-old Singer, but I can't seem to sew a nice zigzag on chiffon.  I'm just trying to finish a seam with zigzag, but the fabric bunches up longitudinally under the zigzag.  I've tried changing the needle size, adjusting the tension.  The old Singer sewed a nice flat zigzag before it died, and I can't figure out why the fancy new machine can't do this.  Does anyone have any suggestions?

Just a thought, but have you tried doing it with a stabilizer underneath the seam?  I know it's a pain, but I have found that these new computerized machines just don't have the stitch quality of the mechanical machines of 30 and 40 years ago. They have fancy features, but so-so stitches.  I often have to use stabilizers.