Run a straight pin under the spring on the bobbin case to remove any lint there. This is easy to do but, if you need a demo, Jamie Wallen shows how to do it in this Youtube video:
http://www.youtube.com/watch?v=Q1mRhcquZTM
I know it's possible for the upper tension spring to go bad, so it may be possible for a bobbin spring to go bad also. You may want to invest in a new bobbin case. It's a good idea to have a spare on hand anyway. I don't know about your machine, but mine takes a standard L-size bobbin and I found the bobbin cases very inexpensive on the Glide thread website (their minimum order is $35, but I love the thread so that's no problem for me!).
Edit: Did you try using a different bobbin? I agree that bobbins can get distorted and then not work properly. The bobbins also need to be wound well; badly wound bobbins can cause all sorts of problems.