I was trying to find gear ratios to figure speeds, thinking you might be hitting a speed limiter. But I can't find ratio's.
Other thing I'm thinking is the turbo. It may be going bad. If it gets too hot the speed of the turbo and subsequent air volume (not boost pressure) may slow. This will heat the air more which will reduce power. The boost pressure may stay near the same though due to the heating.
We don't have the diesel on this side of the pond so these are just best guesses.