my renault clio wont start?

sometimes when i try and start my renault clio grande 1.2 x reg car it wont start.also nearly every time i start it i get a fast reving sound for about 10 seconds then the revs go back to normal.i have had the throttle body changed and renault have had it on the machine but it was there a week and it never not started.they have replaced the fuel sensor on it and said it was that,but after a few days the reving has started again.all this is very intermittent.thanks.