Hey Mick, thanks for your input.
I agree it could look quite terrible that lawn, but the reality is this mower does the same thing on all my lawns at some point, off coarse it will only do it where there are bumps and when there are bumps it does this. I have tried mowing at slow speeds, tried mowing without my back against the back rest, I still get the same thing. I have been mowing on that particular lawn for over 2 yrs now and tried different angles on it but due to the undulations if I mow in any other direction it scalps the lawn fromt he different direction.
Its not specific to the hydros either as I think I have mentioned before that I have tried riing over bumps without my hands on the levers and it makes no difference, I have also tried with just my fingers on levers but doesnt change the ride. So if its the hyrdos influencing the jerkyness, then its not due to my hands... I have had a mower in the past that was jerky on concrete from hyrdos so i know all about that.
I must stress to you, I am not cutting at 15MPH I never do!! On this lawn I would be cutting at less than half speed most of the time, but as I say the jerkynes is there at all speeds!