have you tried sewing with a zigzag?
even a super mild one helps.

What type of machine do you have? it may have a "stretch" stitch on there... =)

Check your sewing machine manual to see what setting it recommends for stretch knits. If it doesn't, the zigzag (as mentioned) will work. Most of your tensions will be between 3-5, but check your bobbin to make sure it has proper tension. I had this same trouble and had my bobbin threaded incorrectly, but if there's improper tension on it, it will affect the thread. To check the bobbin tension, take it out (bobbin in holder), hold it by the end of the string, then give it a gently "drop", keeping your grip on the end of the string. It should drop about 2-4 inches on its own if the tension is good. If it doesn't drop like this, your tension is too tight. If it drops to far (mine fell to the floor!), then it's too loose. In both cases, you'd need to adjust the small screw (loosen or tighten) on the bobbin. Good luck to you!