i would take out the bobbin case, clean it thoroughly with a brush, clean the area that you put the bobbin in, rethread everything and make sure the top thread is going correctly through the tension discs. see if that helps your issue.
if it doesn't, i'd try a different bobbin and see if that helps. sometimes even a little bit of loose thread caught in something can make a mess of things.
if it still doesn't clear up the problem, post again!