low revs when starting Renault Megane???

Just been to test drive a 05 Renault Megane from a private seller, however, when starting the car the revs were really low and had to really accelerate to prevent it fron cutting out. After the test drive I turned off the engine and started it again and the revs were fine. Has anyone got any idea what the problem could be. Debating whether its worth buying or not..help!!!