Sounds like one of the absorbers is jammed. they are simple sliders which suppress the oscillations by the friction of the slider sleeves, flow of damping oil through a restricted valve or restricted airflow within the unit. They are generally mounted with nuts or bolts at the top and bottom of the unit. Remove the one which is giving the problem. Try greasing it if its a non sealed type, otherwise replace it.
Spares suppliers:
www.4directelectricals.co.uk
http://www.ukwhitegoods.co.uk
www.ukapplianceparts.com
http://www.domesticspares.co.uk
http://shop.ezee-fix.co.uk
http://www.espares.co.uk
or simply go into google and type in "washing machine spare parts" - selecting "pages from uk".
Repair and service your own washing machine. Go to www.2ndwave.co.uk/manual.html
dannyboy
November 2008