I was having this same problem. Glad to see so many tips to try!

i have the 1600p and was having trouble one day and did all the above. but i took
it out one last time and it wasn't in all the way. make sure your bobbin is in all
the way and threaded the right way

Is the bobbin the same one you were using or is it a different one that was wound? I have found that some bobbins seem off center when you put it in the bobbin case and thread it and pull the thread does it pull freely? If it does not or the bobbin does not seem to rotate freely I find it is best to just toss it in the trash. Nothing worse than finding when you are having problems with thread breakage and wondering if it is the same bad bobbin you had in weeks or months ago.

I am thinking it is the case. Even if I just stick any bobbin in and just pull on the thread without it going the way it is suppose to the bobbins don't want to spin freely. Not sure how this happened cause I didn't drop it or anything.

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.

I just thought of one more thing, when putting the bobbin in the "case" be sure the thread is turning clockwise when it is being pulled from the case.