Aut Scenic Box 1999

Hello, I would like to know if someone can guide me about the Renault Scenic Automatica 1999, it works but after traveling approximately 45 min remains as neutral, I turn off the car and it seems that when it cools it again grabs the changes. Will it be a serious problem ?? Does anyone know what the problem will be? GRAAAACIAAASSSS FOR YOUR HELP !!!