td5 mis firing

hi can any one show any light to my td5 disco mis firing ,runs okay but has an itermitant mis fire that comes and goes in a split second occasionaly it goes into limp mode and in the past has put engine management light on although when my son put it on a fault reader there wasnt a code he thought could be injectors starting to go,also was told it could be injector loom harness have replaced that to no avail some one else thought it was ecu , but havent got a great deal of cash [ who has ] to throw into things on a might be method