Nissan X-Trail 2 Ltr Diesel 2009 Lack of Power?

My 2009 XTrail has an intermittent problem - after driving from cold after about 20 miles or so, you lose power and therefore acceleration above 2000 revs - its like driving the car with a 50 cc engine. Initial diagnosis on the Nissan computer of the fault code diagnosed the Power Boost sensor, but this didn't fix the problem. I originally had the same problem (although that was constant - i.e. didn't go away after the engine had cooled down) some 3 months ago, which was 'fixed' (after 4-weeks in the garage) by changing the Power Boost Sensor, then the Turbo - both to no avail and finally, after much input from Nissan Technical by replacing a cracked electrical connector. Unfortunately, this problem now appears to have come back (some 4,000 miles and out of Nissan warranty later). Once the fault is sensed the Mal function light comes on and stays on, even when the engine seems to work as advertised - first thing in the morning. You also lose the ability to use Cruise Control when the engine is stuck in this low power mode.
Has anyone any ideas? Are we possibly talking about an ECU change?
Its currently back in the shop undergoing more diagnosis.