Uneven stitches while machine quilting.

The info I found has no mention of the quilting bar for the Siinger 9960 walking foot. Most machine with that functionality include the quilting bar with the walking foot. There also needs to be a hole through the walking foot and a method to attach the bar to the foot which doesn't seem possible with what I located. However, you may be able to attach a bar or a guide of some sort using something like masking tape that can be used as a guide.

On a straight-line quilting tutorial I saw recently, the quilter laid down strips painter's tape to mark the lines on the quilt before doing any stitching. Then she simply stitched next to to painter's tape to get straight lines.

The needle will probably zig-zag, but No the machine generally will not sew sideways unless it is stitching embroidery. To sew sideways, you can use a darning foot or free motion foot, lower the feed dogs, and manually move the fabric sideways as it stitches (see free motion quilting).

https://wpt.org/SewingWithNancy/Video/free-motion-quilting-beginners-part-1

When stitching together a quilt sandwich, it is recommended to use a walking foot on your machine. The walking foot moves the upper fabric in conjunction with the feed dogs. Sometimes, if the quilt sandwich is not too thick, a roller foot can also help but may not be sufficient for a quilt sandwich.
What is Walking Foot Sewing Tips for Beginners
How To Use Walking Foot
How to Use Roller Foot

You may also check the presser foot pressure on your machine. This regulates the amount of pressure the presser foot applies to the fabric and the feed dogs. If the pressure is too strong, it will tend to push the upper layers of fabric, creating a bubble look. If the pressure is too loose, there is not enough pressure for the feed dogs to physically pull entire quilt sandwich under the presser foot.
(The pressure is adjustable depending on the types of fabric and thicknesses being sewn.)
http://www.sewdaily.com/how-to-sew/sewing-machine-basics/the-best-kept-secret-on-your-sewing-machine

http://makeitsew.biz/presser-foot-pressure

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make sure that the thread tensions are correct for the thickness of material
remember that only the bottom foot walks and draws the material through so it may need a help and be pulled as it sews.
I find that women in sewing shops just love to talk about problems and how they can help so find a shop and discuss any problems with them

sounds as if your bobbin is getting caught up not feeding right , also had some luck in past adding clear tape to cover serrated foot for finer fabrics and things like real silks and such but it has to be replaced a lot

The quilted fabric may be too thick for the walking foot to move along smoothly. Try using a zipper foot which is much slimmer and can get close to the line of stitching.

a quilting foot or buy one with a round hole in it great

Hm, just guessing here.

Have you checked to make sure the walking foot (u-shaped) bar is still attached to the needle bar? Are the feed dogs still in the up position and operating like they should?

Learn How to Use Sewing Machine Walking Foot

Something else that I've run across in my sewing adventures is that walking feet need periodic oiling to keep them operating. Just manipulate the walking foot with your hands and watch where there are moving joints. Then apply a small drop of good quality sewing machine oil to each joint. Work it in and then wipe off the excess so it won't stain your fabric.

check with a brother dealer about a walking foot

Usually a walking foot is for straight forward stitching only.It keeps the many layers from shifting for you. The "hand-look" quilting takes a couple stitches forward then one back. If you look close, you will see about every other stitch is thicker and stands out more.