how can i fix my 2001 vectra 2.0 dti?

hi i hav a vecrta 2.0 dti which is fine,i can drive it all day long and its fine but when i hit the motorway and the speedo touches 70mph the emissions light comes on and the car loses power so i have to pull over and turn car off for a minute and then start it back up and everything is fine,has anyone got any ideas please.