GusMac

I have a 3 year old DS4 20 litre diesel sport. It has started to go into a violent vibration when I start the engine so that I immediately switch off the ignition. This only happens when the engine is hot. Very mysterious!!! It feels as though the starter motor is fighting the engine when it fires. I suspect the pinion on the starter is not releasing when the system is hot. Would I be correct in this and if so what is the solution? Fit a new starter? I need to attend to this before real damge is done. Anybody experienced this before me? Thanks