Possibly a problem with the damping system - shock absorbers at the bottom of the drum which are meant to minimize oscillations. Theyr'e 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.
Check out my site for info: www.2ndwave.co.uk/manual.html
dan, February 2008