The rubber feet on the right hand side of the tray were resting lightly on top of the metal brace - that surely can't be right, as the right hand side would bounce off the underside of the lid during any vibration. This would accurately explain the skipping behaviour. As a short-term fix I have taped the drive tray to the right hand side. I took the car out for a 1-hour drive on the motorway this morning and the problem has entirely disappeared!