Hello Lucy,
Does the machine sew in straight stitch ? providing nothing is loose on the top of the machine it should sew perfectly, move the zig zag lever back and forth rapidly whilst the machine is running slowly, failing that it can only be broken teeth on the upper nylon gear which is on the top shaft, it should be an off white gear, if that is the problem then it is a shop repair and you may find that if that is the problem the repair charge may far outway the value of the machine, try the above as a last resort let me know if it is cured or not.
Ken
Ken Dourof, June 2007