You have something wrong there. It's almost never the machine that is wrong, it's almost always operator error, but a good idea would be to replace the bobbin case or the spring steel tension blade on the bobbin case. In the mean time, I would loosen the knob on the top thread tension and tighten the bobbin tension to even things out. It doesn't take much adjustment on the bobbin tension to tighten it significantly, a half turn of the small forward screw is a lot. Also, check to see the rear screw is tight. If that screw is loose it can really drive you crazy.
The picture shows where my top tension knob is situated. Start from there and adjust the bottom tension accordingly.
No one lives forever, the trick is creating something that will.