Maybe you need to make your stitch length a little shorter. When I'm piecing, I set my length at 2.
It's quite alright for you to adjust your upper tension -- that's why it's adjustable. However, it won't do any good to adjust the bobbin case tension on that particular machine. Try sewing with a dark thread on top and a light thread on the bottom and sew a zig-zag stitch. With a zig-zag stitch, you can see better what the tension is doing. If the dark thread is pulling to the back, tighten the upper tension. If the light thread is pulling to the top, loosen the upper tension. Different threads may need different tensions. I find that polyester thread needs a tighter tension than cotton.
Gook Luck!