xantia110 HDI low revs misfire

[exboy65a]

With 150 000miles I amgetting a misfire ----feels like its on 3 cylinders below 15---2000 revs. It did this before last year and cured itself. On sunday i did 55o miles and never missed a beat, plenty power no smoke no excessive fuel consumption. Worst on cold start up, almost feels like electrical, considering a filter change and dosing with fuel additive------could of course have been my last tank of fuel. Any ideas, sensors failing injector ?????????. I live in France and of the 4 Xantias in my local Scrap yard they all have 450000 kms on. Hope for us all yet!