Assuming this is an older car and you have no maintenance records. How about checking for chain stretch, chain guides and tensioner and injection pump timing? This proccedure also covers cam timing as well. Or do you know those items where addressed at some point in the last say 60k miles? They should be checked anyways if you have no knowledge about your car even if no obvious trouble exists.
Next would be to check the injectors by removal and having them checked for pressure, spray pattern and leakdown. I might even squeeze a compression check in between the two above items.
Make absolutly sure you are not getting any air into the fuel first of all. The idle will be rough if you see a small stream of bubbles being processed through the primary filter for example. The heat on rubber hoses has to be hard on them where you live. Or duplicate the setup if you directly fed the injection cleaner into the system only using diesel oil in the container to get absolutly clear of any air leaks as a source of your problem. You can also do an injector line loosening check as described in the last post or so to establish if all cylinders appear equal as an early step as well. I think you will find it one way or another.