2002 renault megan dynamique convertible - engine/auto light problems?

I have a 2002 Renault Megan dynamique convertible which is automatic. Every now and then the light showing "an engine with auto underneath" keeps coming up. It does not flicker but is on permanently for short periods of time. It happened again this morning without warning and the car looses power and struggles, despite putting your foot on the accelarator. I've just checked it in its stationary positon and there is no light, but may come on later whilst driving. What can this be - does anyone know? Many thanks