First questions always good to ask. Did this just start doing this toady? Is this truck new to you or do you have any history? Any others of same vintage at the location to compare to it(or steal parts from!)? How about a S/N to help if someone wants to look on SIS for ideas?
Been over a year since I worked on or even saw the 769D at work, retired. Anyhow I seem to recall that there is something in the computer that reduces power and RPMs until engine reaches a set temperature. I recall ours was a major dog the first couple runs in the morning. Might be worth checking into what senders control that system. Thinking it might be a bad sensor telling the computer it's not warmed up yet. Is this an electronic engine or an early 769D with a HEUI Fuel System?
Have you tried doing any calibrations, don't recall what is available with ET for this truck, maybe throttle position sensor? Are brakes releasing right, should roll easy in neutral on slight grade. How does it do running empty on level ground, does it go through all the gears?