I have exactly the same problem, apparently it's a faulty sensor on the gearbox. you will have to put it on a diagnostic machine to identify which one. Remember if it works sometimes it's not mechanical but electrical, so don't get conned into having the gearbox rebuilt. Don't go to a Renault dealer, find a good gearbox specialist, they are far better. I've put up with the problem for a few months now, puts the fuel consumption up a bit on the motorway!
Steve
April 2009