Renault Megane miss in engine

Last year my 2004 Renault Megane Turbo Spots started to play up with a sorta miss in the engine when at a constant speed. It's not there on acceleration, only when cruising. I have had four mechanics check it, but none could find anything wrong. They changed the alternator and the exhaust sensor, the injectors were cleaned three times, new spark plugs were installed, had the vacuum test and the camera up the exhaust pipe. Don't know why they did that. One said it was a sticking exhaust valve, but that would cost $4000 and it might not fix it! I'm out of ideas. I don't want to get rid of it as I love it, I was also insulted with a car yard offering me $5000 for it. What can I do?