Now I know the lags are level and secure - I have gentely moved the drum and find the is alot of movement front and back - I have opened the top and from what i can see both spring are attached either side - other than that I am a little lost....
Probably fault in the suspension damping system - either a shock absorber type component or a friction pad system. located in the base of the machine. it minimizes oscillations of the drum. when its faulty the spring oscillations add together and allow the machine to jump. check out www.2ndwave.co.uk/manual.html
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Try fixfault.com to fix washing machines.