It's possible that the injector nozzles are bunged up, but I'd also suspect your injection pump delivery valves may need some new shims and o-rings...followed by Diesel Purge and an Italian tuneup. These engines are fairly simple, so there's just a small number of items that can go wrong to produce the symptoms you've described. A compression test would also be helpful, in order to rule out the truly nasty things that can contribute to your problems.

You could also help to isolate which cylinder is causing your idle problems by running the engine and cracking open the metal injector line at the top of each injector (14mm wrench). The engine should run rougher whenever a good cylinder's line is cracked--the results will be less noticeable when a bad cylinder's line is cracked.