Nissan Navara?

My 2002 D22 3L Turbo Diesel is over revving this only occurs when it is warm and also the motor cuts out doing highway speeds approx. 90Ks/ph. It seems to do this on a down hill run. This problem happened previously when vehicle was still under warranty, they fobbed me off with an excuse then. The utility has just done 110,000Ks and was running well until this happened. Any ideas and what can I do to fix it. Cheers