how can i fix my megane 1.4?

Hi i have bought a Megane 1.4 54 plate 3 months ago, Since i bought it, i have noticed that when the revs have dropped below 1000 after starting, the engine conks out. It starts again and then engine stops again, this could happen another couple of times before stabilising itself and reving it for a few seconds and then i sometimes get a smell of petrol, this problem happens nearly everytime i start the engine with the key card, i have had it in the garage twice and they have changed different sensors on it, the problem seems to be getting worse, please can anyone give me any ideas what it could be.