I'd try changing the needle and making sure that the bobbin race is threaded correctly. Sometimes, if your needle is nicked it can cause problems underneath. One of the things I learned from working at the sewing machine dealer was that when a seam is sewn and it's messed up on top then it's the bobbin thread that's causing the problem. If it's messed up on the bottom then the top thread is the culprit.
is that true? I have been having trouble with my thread bunching up and jamming up the machine on the bottom. I thought it was a problem underneath. hmmm I'll have to pull it out again and try playing with it more..thanks!