With the 6th gear issue your mentioning, I'd of thought something along the lines of clutch or gearbox related issues. If its still in warranty then I'd pop it down to Renault for a look or if not then pop the bonnet up and have a good look around when you get a wife of the smell
Surely if it was a gearbox issue then it would be when I am putting it into 6th. The car has now issue going into gear at all 6th is struggling when I'm just driving along