Possibly a problem with the damping system - shock absorbers at the bottom of the drum which are meant to minimize oscillations. They're two main types, either sliding pistons or a sprung friction pads which bear onto friction plates welded to the drum. Whichever type they are, they're probably worn out and needing replaced. They suppress the oscillations of the drum when in spin mode so the machine doesnt hop around the floor. Friction dampers are like little square brake pads attached to sprung arms mounted on the machine frame which bear onto friction plates attached to the drum. Any excess movement of the drum is dissipated by friction between the two surfaces. If they are the friction type, prise the sprung arms apart with a length of wood, and replace the pads.
The other type is a sliding damper, which is like a scaled down version of a shock absorber in a car. Some have inline springs to support the drum, but mostly 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.
Repair and service your own washing machine. Go to www.2ndwave.co.uk/manual.html
dannyboy
January 2009