Renault Megane trying to start whilst driving!?

Having a big problem with the wife's Renault Megane (1.9 tdci, Estate - '57 plate).
Whilst driving at any speed, there is a jolt (sometimes small, sometimes big) and a message on the dash to put in neutral.
Spoke to a Renault mechanic show said it sounds like the engine is trying to start. The starter is not engaging as their is not grinding/crunching noise.

Car is currently with a garage who cannot replicate this fault on their test drives, but any ideas as to what it could possibly be. I guess the car thinks it is moving but not realising the engine is already running!