vauxhall signum 3.0 cdti loss of power?

please, please, please can anyone help me. I have a vauxhall signum 3.0 cdti which loses power when i put my foot down. It seems to be ok when I slow start but as soon as it gets to around 2500 revs, it loses complete power for 5 seconds or so. If I keep my foot totally to the ground, it accelerates eventually but seems to shudder as power is lost again. Just replace air flow system but problem is not solved, its exactly the same. Been to an auto electrician and it shows nothing up on the diagnostic machine. Please can anyone help!!!