I came across this site about sewing machines being jammed and other helpful topics. I thought maybe you should check it out. It might have something useful and it might not.
Here is the sight for you, Miranda:
http://www.thriftyfun.com/tf45666588.tip.html
And here is a site that has all kinds of different topics about sewing machine trouble. It has the topics listed as soon as the page comes up:
http://sewvacdoctor.com/cate.asp?categoryID=11