Hi Graeme
Have you got a diag, are you guesses it's the gear box? so you loose power at high speed and higher revs?
This could be a turbo issue ( http://www.ebay.com.au/gds/Signs-That-Your-Turbo-Is-Failing-/10000000204969079/g.html) , so it could be going to limp mode ( https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=limp+home+mode )
Hope this helps and report back :)
Regards
Bradley