im not the expert here, OP says loops on top. if you get loops on top it's the bobbin tension is significantly lighter than the top. Does the bobbin case have a deformed leaf spring? have you carefully so as to not loose the 2 tiny screws that hold it on and make sure there is no buildup holding it out? try the yo yo test to see if it drops, but only a little. are you using same thread top and bottom.