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!!!

sk
August 2009