I have an A5 Cabriolet Diesel Turbo 3.0 and the glow plugs ()came on about a week ago , followed by the Engine light a few days later. Starting the vehicle is difficult for the car just refuses to kick over and the acceleration in the car has vanished but the car will pick up momentum and still able to reach speeds required albeit it now takes a 1/4 mile. The taco also will not rev any higher than 3 What do I need to do.. Please..