2002 Renault Laguna IDOL SPEED

Hi, I have taken this laguna back to renault 8 times, the car on pulling away in first gear,first vibrates then the revs drop so to compinsate I have to press on the gas and bring the revs up tu about 600 revs above the idol speed for the car to move on,now renault have pluged it in to a computer,and could not find anything wrong, so the took the clutch out and said it was fine,now recently someone said that the gear box needs oil, i taught renault would have checked that,but maby not, someone said it might be a faulty idol sensor. If you could offer any advice it would be much appreciated,thank you for your time, Regards pete.