I had tension issues at first as well, not with piecing, but on the frame. Mine seems to do well with polyester thread, and I use pre-filled bobbins usually, with no trouble at all. I needed to adjust the bottom tension on mine, and it took 1/2 a practice quilt before I got it right, but once I set it, I have done 6 more quilts with no problem, and taken it off to piece in between. If you have a repair guy you trust nearby, I would even take it to him/her to help you- for me it would be worth the cost of a visit.
Also, make sure she is oiled frequently, including a drop in the tension disk once in a while, and swab it with a q-tip. Mine acts up when she is dirty or thirsty. But I wouldn't trade her for the world. Once you make friends, you will never look back!