Renault megane grand scenic 1.6 2007

My grand scenic has started to misfire and also lose power and cut out. It has been into garage several times as the fault isn't happening all the times but nothing shows on diagnostic. I can drive the car for two weeks and nothing happens and then it could happen 4 or 5 days in a run. The local garage have suggested throttle sensor but they don't want to change until their 100%, before we had these problems we had a cam belt change and someone else has suggested a cam belt sensor but nothing will show up on diagnostic.



The car is now booked to go into renault on Wednesday but not sure that anything is going to show up without it costing loads. Any ideas?