Freelander 1 intermittent loss of power after 30kms

ongoing problem 2006td4.car starts and runs great losts of power.only when been on a trip of around 30klms and about 90kms per hr. i start to slow down a little the car begins to fill like its missing and drives very lumpy.put a little. power on again and all is ok.have had this prob. now for a long time now.any ideas please.no warning lights on and have the normal checks and diog.done help please

You need to b checking the live engine data for anything changing as it drives. Look at low pressure and hign pressure fuel supply, MAP signal and and MAF signal.