2016 ISX15 Cummins Fuel issue?

[steeldck]

I have a 2016 389 Peterbilt with a ISX15 cummins. this truck is deleted. Started out with the truck acting as though it gelled up during our cold spell. I took the truck to our local cummins guys shop here, He plugged it in and had the dreaded 559 code, ran some tests, found injectors passed, said it was the fuel pump, so he replaced it.
Truck ran fine for 2 or 3 days, then the problem came back, throwing the same code. I looked into a few things, changed filters again, and took it for a ride (bobtail). the problem came back, with the code ,around 30 minutes into the drive. It acts as though you are slowly turning a ball valve off for the fuel, it wont completely die, but barley will will go down the road, seems you can pedal it a bit, and kinda get it going again, but its not right.
I brought it back, changed the poppet valve on the rail, as well as the rail sensor, same result, only now, no code.
came back, checked stand pipes in both tanks, checked vents in tanks ( can blow right thru them), plugged off fuel lines and ran on each tank individually, with the same result each time, 30 minutes or so in, it acts up, again, no codes. checked Davco filter out, pulled the lines and fittings off and checked to make sure there was nothing blocking them, nothing. I cant physically see anything leaking either. I pulled the banjo fitting off the pump, to check if there was anything in the screen there, nothing. Finally took the ECM out of my other truck ( twin truck, 2 serial #s apart) put it in this one, and test drove, same result.
Had a tech come and plug truck in, ran injector test( a bit violent) and they all passed. we left his computer hooked to the rig, and went for a ride, an hour plus, stopped, shut truck off and rolled on, do you think this dang thing would act up, nope, ran great the whole time. he un hooked and left. I let the truck run on high idle( 1000 rpm) in front of my shop, 20 mins, and it completely died! Started it back up, put it inside. Im at my witts end here! Please Help!!
thanks