2004 X Trail Sport dci uneven tickover and coarse acceleration?

My 64000 mile fully serviced X Trail began without prior warning to tick over unevenly and accelerate coarsely below 2000 revs. Over 2000 revs it runs smoothly. Have had diesel pump checked, injectors checked, pressure valve in common rail replaced, ECU software checked, engine compression checked and numerous attempts at diagnosis by Nissan main dealers. The problem persists. Anyone had similar and, if so, what was the solution please ?