Try cleaning out the bobbin area really good. Sometimes a little piece of thread can cause a mountain of trouble. Try a different bobbin, a new needle, different thread.
Make sure you have the bobbin case in right. I had mine in just a little off one time and ended up taking it in to have it fixed because it just wouldn't sew right. Turned out I had the bobbin case off just a wee bit. Boy was I embarrassed. Glad it something that simple though.
Hope you get it solved. :D :D :D