Renault Megane 2004 Problems

Your description of the problem suggests it's not an ignition problem, so changing plugs etc was probably a waste of time and money. I'd be investigating the fuel side of the equation, looking for things that might be making it lean at low throttle openings as you would be when cruising. Is there an air leak that's affecting the mixture strength, or something like that. Having said that, it could be a sticking valve as the mechanic has diagnosed, but I would expect to be able to see that on a compression test. If it is a sticking valve the only to know for sure is to take the head off and physically check it.