Cat D9G Power?

Hello George, better to send us a video so we can hear what it is or is not doing.

as Rome suggested, a video would be best....we like videos..

-since we're talking diesel power, my first wild guess would be fuel system....lack of fuel...fuel restriction of some type, lines, filter(s) etc...

Birds built a nest in your intake?

High idle rpm is 1455 +-30 rpm, 475 +-10 rpm, set decelerator at 700 +-50 rpm
I take it you already checked the rack and fuel pump lifter settings? If I remember that was an ordeal.
Fuel pressure good?
Check rpm at hour meter, that is half engine speed, or use a laser tach.

Rome, is that decelerator setting the rpm that the engine should be at when the decelerator is fully depressed? In gear or neutral?

Fully depressed, neutral.

I will be taking some video later today... Actually my wife will.... Yes, the rack was the first problem and then the rest.... Fuel pressure looks good starts right up... I am thinking maybe decelerator lever as engine just does not come to life with throttle fully open and decelerator engaged. When I lift my foot back engine does increase RPM of course in Forward 1 she moves and all just hard to get engine winding up. Thats why I thought torque converter first... I will take videos and post later today. Remember this is Hawaii so expect them tomorrow...

Easy check would be that the decelerator really is releasing all the way and not holding back due to binding or interference. Does the engine go all the way to high idle in neutral? It can go to high idle but not be getting full fuel under load, which "usually" is because the rack is not opening all the way up under full load but it could also be intake restriction. If restricted, it would make black smoke (all the time, not just the rolling coal at acceleration : ). Could also be exhaust restriction.

How much oil in the converter case after shut down? There should not be much, if it is flooded it will act like a brake. This can be caused by converter failure, or scavenge pump failure or a blockage. Take a bucket underneath, you should only get about a gallon if all is ok.
best of luck, Wombat