I have this machine and love it, it is a work horse. The only thread it is not fond of is coats and clark. When I use that thread I have to adjust the tension. Make sure you check the tension on the bobbin. Tighten or loosen the screw on bobbin. The bobbin should barely lift up from your hand when pulling the thread.
I use all sorts of thread on this machine but it likes Connecting threads, Embroidery thread from Thread Arts and I now use bobbin thread that is 60 wt. I love using this.