You should get a can of diesel purge and follow the directions carefully. Basically you run it full strength through the fuel filler and into the IP, with the fuel return line running back into the container, until the entire can has been used. I do this once a year with both of mine and they always show substantial improvement after the purge. Just using fuel additives is not nearly as effective.
Don't get oversold on that damper bolt. If it is set right and is doing what it is supposed to, that problem is solved, until the bolt gets worn futher. You already said the engine smoothed out when you set it. That is a very good result. There are a host of other things that will make these old diesels shake and so on. Just solve one at a time.