I've found that when that happens to me, it's usually the type of needle I'm using with the thread I've chosen. As Kathy suggested, check your tensions (both top and bobbin) to make sure that they're feeding evenly and smoothly. What type of thread are you using? If it's a 50 wt or higher, try a microtex sharp 70/10. If it's a heavier weight, then a quilting needle or a top stitch needle 18(110) might work better. I've found that when using regular Coats and Clark cotton covered poly thread, a universal 90/14 or 80/12 actually works well.
This same thing just happened to me when I started on that queen quilt I was basting, and I didn't take the time to do a practice swatch (BAD lin). I was in a hurry to start, so I had a few stitches to take out. (served me right! LOL) I still had my regular Juki needle in the machine, and I'm using a 60 wt thread, so I changed to the microtex and now I'm having no problem. Over time, I've gotten a pretty good sense of what needle will work with the threads I most commonly use, but that's no excuse for me to have not done a practice swatch. If I had done one, I would have known immediately that I hadn't changed out the needle.
I had a lot of fun practicing with different threads and different needles. I made a little chart as I went along and marked down which needles I had used to get the best results with any given thread. This can really be a help to you. Also, when doing a practice swatch before getting to the actual quilt, its best to use the same weight fabric (if not the actual fabric(s) used in the quilt) and a small piece of the batting you'll be using. It's probably most important to at least make sure you're using the same batting in your sample. Different batting weights can come into play as to how the needle reacts. Good luck. Don't give up. I'm so glad I didn't. :)