2009 cm 871 has us stumped

[BusterB]

Greetings
Brand new and desperate, I have a 2009 Pete 367, with a long deleted cm871.Truck ran like a champ for years then one day she started cutting out and eventually bricked the original ECM.
Put a Cummins reman ecm in and she fired an purred as usual. Road tested all good!
Next day no lift pump no milage or hours.Unplug the Power at the ecm and replug, and the lift pump kick in and she fires. Then 20 min later she stumbles and dies. Restart and idle up for 20 she stumbles and dies.
This has been going on for two weeks.Harness has been stripped back and inspected, all new relays, batterys and cables even put a second new ecm on. Yesterday key on for 30 min no pump then the pump randomly kicks in as im walking up to the truck. Runs on hi idle for 2 hours perfectly. Shut off and go home. Today no pump no odo or hours. do i need holy water?