John Deere 450DLC excavator fuel issue

so I have this 450DLC in my fleet with 9800 hours on the clock with isusu 6w1g tier 3 power. code 1239.01 or p1094 for isusu, came on book says no pump pressure stage one. I started with changing fuel filters, blew back feed and return lines, checked strainer in the banjo bolt on transfer pump, drained fuel tank and pulled down the sump to check for debris, found nothing and code still active. next I tapped in fuel pressure gauge onto transfer pump, 30psi at idle, 45 at ft, started digging and after an hour the fuel pressure starts dropping got down to 32psi then I pinched the return line and pressure stays the same, code comes on, ok so I change the fuel supply pump, which comes with the transfer pump, and overflow check valves on pump and secondary fuel filters. put it back into service and two weeks later the code returns, this time fuel pressure is good no drop at all, replaced common rail pressure sensor out of desperation with no change. my next step is to try and see actual and desired rail pressure but I don't have access to the software, any thoughts?, did I miss something?

John Deere 450DLC excavator fuel issue