It's been a while since I checked this site, so I'll fill you in on my situation with my "Knock knock"...
After removing initially removing the injectors, having them cleaned and rebuilt at a local diesel machine shop and replacing them, the problem came back rather quickly, as posted last year.
I searched high and low for a mechanic to take a look, and was surprised that all of the local tractor shops only worked on "their" brands, the auto garages didn't want to look either. (I wonder if that has changed due the current state of the economy?) Finally, I was directed to a mechanic that works for a construction equipment rental store. I thought that he must be pretty good, since all of their rental equipment is older than God, and still works fine.
His first thought was injectors also, and removed them, took them back to the same machine shop. They found that one hadn't been re-assembled correctly, so they fixed it. I ended up paying much more than I should have, but have a fine running tractor again.
I guess the moral of the story is that you can't always trust the "professional".
