I was very dismayed when I started testing out my new Cs6000i and discovered I was having the same thread tension problems people complained about. After several hours adjusting the tension to every possible setting on several different stitches, it seemed to be the bottom thread that was the culprit.
I decided it might be the flimsy plastic bobbin---when I switched to a heavier duty metal bobbin of the same size---IT WAS MAGIC! The machine started making perfect stitches on the factory setting of tension at 4!!
If you are having the same problem--it's worth a try! Hope this helps someone!!