I have, of late, noticed a pronounced judder on start-off. It happens as I push the pedal down the first time to move off. The motor judders quite violently several times, sometimes shaking the whole bike, especially so if I maintain a light pressure on the pedal with the brake on, however the effect goes as soon as I am moving and doesn't reoccur if I slow down and then accelerate, only if I stop and start. On loose surfaces the judder causes the wheel to kick, fun on mud! In other circumstances I would say that the chain is jumping the rear sprocket (it's not!) as that's the feeling I get. Any ideas?