Hello Maz,
First of all the 237 is a good machine and you do not need to spend any money on that machine at all, a service consists of a few drops of oil on all moving parts that you can see and keeping the machine clean, if youi have put a new needle in and the machine is skipping stitches then you need to retime it, you can do this yourself it is fairly straight forward, a bit of patience and you can do it, I can attach timing instructions for you if you e-mail.
Ken
drivewithken@blueyonder.co.uk
August 2008