Do you just want a smaller sewing machine so you can take it with you for quilting guild/group activities? Or will it stay at home?
If it will be staying at home, then before getting a new machine maybe try getting out your manual and following the directions rethread the top and bobbin. Then stitch only a few stitches to see if you still have that problem. If you still have a problem there are several posts on the QB as to how to adjust the tension; there are several tutes on youtube; and, if possible go back to your local dealer and have them give you a hands on tute on your machine.
One friend actually had a defective machine which the dealer replaced. Another friend found she threaded the machine wrong, doing what she had done on another machine didn't work on her new machine.