(I've just fixed mine so its probably too late for you but may be useful to others). Take the lid off (2 screws at the back and pull towards the back hard!). You will find a "Breeze Block" type weight on top of the drum with three bolts. Tighten the bolts to secure the weight to the drum. Check the top two springs and the hydraulic struts at the bottom (you will need to take the back off to check those).
Carl Cruz, August 2006