Hello Helena,
Your machine needs retiming, you can do it yourself if you can get to the bobbin case area, I can attach timing instructions if you email me but you will have to get my address from any answer from page 25 down because this site no longer allows email addresses to be shown or type my address as you say it not as you read it, name as below
then, at blueyonder dot co dot uk
drivewithken, May 2009