I was having that problem a couple of weeks ago. Annette (Quilted Sunshine) said to get Gutermann thread (or another brand). She said you cannot get a good tension balance with Coats and Clark. I was already using Gutermann in the top and coats and Clark in the bottom. I bought the Gutermann thread at JoAnn's and finally got the tension balanced. It took a while. I kept changing the bobbin tension , then the top tension until it was balanced. I used a quilt mock up to keep testing it.
I have not yet conquered quilting with rulers, but I did get that problem solved.
bkay