Avensis D4D lack of acceleration?

My 53 plate Avensis diesel has suddenly started having acceleration problems. It seems okay when cold but when i'm on the motorway doing 55 - 70mph and the engine is at full temperature it will not accelerate. I need to drop it down a gear until the turbo kicks in at approx 2400rpm.
It has done 93000 miles (full Toyota service history) and has recently had the fuel filter replaced and injector cleaner added to the fuel (twice).
The problem is intermittent, and the car idles beautifully.
Does anybody have any other suggestions that I may have overlooked.