Pfaff 7570 bobbin tension

Question

I have owned this machine for several years and this is the first time I have had a problem with the bobbin tension. It seems to be tight enough when I **** it in fact it seems almost to tight but when I sew a straight test strip the top stitches are looped underneath and you can pull the bottom thread straight out. If you do more than 3 inches it will start to gather and then break the bobbin thread. Is this adjustable or am I going to have to get a new bobbin? I haven't sewn in a few months but I would not think that would be the problem. I have also cleaned and oiled it. Thanks for your help. Sandy