P38 Diesel 1996 2.5DT Surge jerk misfire-help

My 2.5 DT BMW motor with manual gearbox starts no problem from cold,runs cleanly for approx five minutes but then begins to jerk when trying to maintain steady speed but accelerates OK.This happens up to approx 50MPH but is smooth above this speed(at its worst at 20 - 30 MPH).Rough uneven running appears to happen mainly below 2000 rpm.Motor will rev out to max revs no problem.I have replaced the fuel pump in tank and this has made no difference.I have also checked the injector return pipes and these do not apppear to be leaking.I cannot see any air bubbles in the clear section of the diesel feed pipe between filter and the pump.I have also replaced the fuel filter.Any help or feedback with this problem would be much appreciated as I am running out of ideas to check.Please help