whats wrong with my rover 25?

hi i have a rover 25 1.4. i was driving on the motorway when the car got to 50mph it then lost power and went down to 30mph however after a while i got it back to 50mph. the car still turns over first time however upon closer inspection there is a black crust forming on the metal that you see when you lift the oil cap. does anyone have any ideas what might be wrong