I will reply to each of these........
black_ You have home work to do. The last thing to condem is an injection pump so check everything leading up to that. Fuel leaks, lift pump supply psi, throttle linkages etc. even the TV Cable op.
Do you know Piers Diesel? They be near you. They know the first gens REAL well!
The pump can begin self destruct mode and act as you described. If you catch the problem early, you can save it and about a thousand $ with a seal kit and some internal springs.
To say that HP is not effected: if you have factory HP then you likely will not notice. If you have engine performance where it should be (400-450 HP) you will notice a difference.
...and Ducky. If your engine runs fine and seems loapy at slow idle speed, your injectors are out of sorts. This could be wear, dirty, OR new injectors that are not set to the same bar (pressure) They must all be the same or very close to one another. :thumbsup
It pays to have the injectors set by a shop. Sure,,,,,, new injectors are a help, but if they don't calibrate them to all the same pop-off bar, then they never will act right. :thumbsup :$:
Scott