still sounds like an airflow fault I have had on many Mitsubishi fg25n.
if I recall the error code is e21 but does not always show. check the wiring at air flow sensor plug back over engine to ecm in the left chassis, if its an older truck this wiring can get brittle with heat/vibration/age.
do you have uptime software (again a mitsi version software but works on the cat as its the same truck) or whatever the cat software is called.
id check that the ecm is seeing an input from maf, id check all engine inputs anyway, id check gas pressure which should be around 25kpa, id check air cleaner and the hose that comes from the air cleaner to the gas unit primary chamber cause if this gets blocked then the diaphragm will not work correctly and bog down like this symptom.
could be the small filter under the gas pressure sensor blocked,
could be an electric throttle problem.

that should be enough to check and eliminate some things and if I think of anything else ill be back