discovery td5

hi all i have a 1999 td5 discovery with very low miles and just wondered if any one's heard of a problem that im getting with it,in 3rd 4th and 5th gear under heavy load it missfires and gives a nasty jerk,this only happens above 2000rpm,i have no fault codes coming upany one have any clue????

Either an air flow meter faulty or its overboosting, check the wastegate is free and make sure all the small pipes are connected to the wastegate and the modulator valve.