Pam,
Since you can sew straight stitches fine, my first though is that you may be moving the fabric too quickly for the sewing speed being used. The stitches are not being completely sewn, before the fabric is moved and causing the rats nest of threads on the back.
You could try setting your machine for straight sewing and just drop the feed dogs. This would eliminate all the machine settings from the equation. The only variable would be the fabric movement.
Also, on my Viking Platinum, there are 2 ways to wind the bobbin. One method goes through the needle. The other is the normal top of the machine method. I'm guessing you use the top of the machine method. Most people do. Perhaps using the other method will give better results.