Sounds like it needs retiming or perhaps the bottom gear has broken, either way you can do it yourself, if you do need a gear they are only about £4.00 or about $8.00, if you e-mail me below I will attach timing instructions for you and if you do find you have a borken gear let me know what country you are in and I will attach a list of sites for spares.
Ken
drivewithken@blueyonder.co.uk, October 2008