I know it has been well maintained but when have the valves been adjusted last? Also a quick check of milli volts on the harness plug might indicate the cylinder with a problem if there is one. If not a cylinder it will show with all voltages being about the same usually. If there is one with low voltage just switch the injector with the next cylinder. Does the problem move? If so get the bad injector checked . If the problem persists on the same cylinder do a compression check on it. Your description could be many things but possibly a cylinder not firing properly is a good place to look. Or thats what I would do first as it is so fast and easy to check. You require a meter with a milli volt function as your glow harness reading will be about 10 milli volts on each glow plug at the harness contacts. With the engine warmed up and running. Edit: re read your post. Sounds like bad injector on rethinking. Too much smoke to be underfuelled.