Hi, this will be no help but I have the exact same problem with my 58 reg which I have had from new, my dealers said they could not find anything wrong with it when under warranty, they said that all they did was plug it in the diagnostic machine and no fault found.
I have a 2008 2.0 diesel done 50,000 miles it has started to pull back, this happens when accelerating hard through the gears at around 2500-3000rpm doing between 60-80mph its more apparent on an incline than the flat, it almost feels like there is a blockage. Any ideas.