probably a problem with the damping system - shock absorbers at the bottom of the machine which are meant to minimize oscillations. Theyr'e either sliding types or a sprung friction pad system. Probably worn out and needing replaced.
Check out this site for spares : www.4directelectricals.co.uk
Check out this site for info: www.2ndwave.co.uk/manual.html
dan
February 2008