how can i fix my boxer 2.2hdi?

this fault appeared approx 5 miles after i filled up,i lost throttle response but van would idle perfect,it will rev up if you give it full throttle straight from idle but gradual throttle it misfires and will not rev passed 2000rpm,it's as if something is holding it back,it has been on a diagnostic machine and the ecu has no recorded faults.i have changed the fuel filter and air filter and also fitted a new mass air sensor all have made no difference,does anyone have any suggestions?

steve
April 2009