why does the auto light come on on my Renault megane expression auto?

When I try and pick up speed the yellow auto symbol/light on the rev counter panel suddenly comes on, it is quite frequent, the vehicle starts to lose power and I need to usually put my foot down and use twice as much petrol to pick up half the speed. The engine also becomes noisier. If I turn the engine off and restart car it resolves itself.
its a 2002 automatic Renault megane expression.