Diesel, power loss, limp mode

[huntertec]

I have a 05 X5 diesel and have had no luck with this one despite exhaustive efforts. Problem in sequence when I'm accelerating the engine loses boost, a dull siren sound starts on lower right side of engine, transmission won't kick down, if I continue to accelerate the siren sound gets louder and then the vehicle goes into limp mode. I stop turn off the engine wait 10 seconds restart the engine and its good as gold, I can accelerate hard and no problem. So the problem isn't always there however it is becoming more frequent.You can imagine how dangerous this can be when overtaking. Any clues much appreciated.