I have just this minute got my car back from the garage, and I have the same problem as you. See my thread: http://www.renaultforums.co.uk/showthread.php?t=1704
The mechanic fitted it up to the diagnostic machine, but it showed no fault. He checked everything but the car looks good, but apperantly, a lot of Renaults have this problem. He says that often unplugging a lead (I think the one from the crankshaft to
) and cleaning it solves the problem.
The immobiliser could also be the problem, as it sometimes can't read the key code. It's sounds silly, but if you've got a spare key, try using that one instead.