My family gave me a Brother (Quilter's machine) for my birthday once and we had a love/hate relationship from day one. It never worked well. I ended up selling it to the guy who tried to fix it, for parts. I now have a great Bernina that I love, but before that I had a Bernina that I got refurbished and lasted a long time. I also had a nice machine I got in a yard sale and now my daughter uses it for when she needs to sew anything.
I believe you need to work with your tension. You are using good threads and most likely the problem is the tension or the bobin.
Where are you located?
Maria