2006 Mitsubishi Triton GLX-R 3.2-litre diesel

In the last month my 2006 Mitsubishi Triton GLX-R 3.2-litre diesel has begun to surge around 2500 rpm - especially from second to fourth gear. Driving to Ballarat from Melbourne recently, when going uphill doing 110km/h on cruise control, it only could manage 80-100km/h! It was struggling to do 80km/h at one stage, and I thought it was going to die on me. My dealer said it's likely to be carbon build-up in the inlet manifold and EGR valve and they would need a few days to repair it. What do you think?