Diesel Bombers - 99 Chevy 6.5L TD Stalling

Hi Folks!
I am a lifelong backyard mechanic without much diesel experience except for a backhoe and this newly acquired 1999 Chevy dump truck w/ 6.5L TD. The truck runs well when it runs, but suddenly starts to lose power, sputter and eventually stall as though it has been starved for fuel. Indeed, when I attach my pressure gauge to the filter drain line, I observe sometimes as much as 5 psi steady, but then at some seemingly random moment it will drop and even go into negative pressure as it sputters and stalls. I have also attached a volt meter back-probed into the center terminal of the OPS wire - Wire Harness plug and observed battery current maintained the entire time the engine is running, which I believe rules out the OPS. Also, I have replaced the lift pump and verified the pressure (more than 10psi) and flow at the entrance to the fuel filter. I have removed, cleaned and replaced the fuel filter canister. I have replaced and remounted the PMD/FSD. In August, I replaced the fuel tank and sending unit.
Yesterday, after a test drive and limping home of over 2 hours, I decided to test pressure and flow directly at the fuel feed where it enters the filter. I noticed that at some point that line was replaced and although it was in like new condition, it was bent severely as it was too long, so I thought maybe it was kinking. Figuring I had finally found the elusive trouble and how simple a fix, I cut off a couple inches and reattached it without the bend. Started the truck, pressure gauge on the drain line,T valve open, 5 psi steady, idled 10 minutes, 5 psi steady. Drove it around backyard like 6 times performing beautifully then suddenly sputtering and sputtering and stalled. I am at my wits end. Not sure how the system can go from running with 5 psi in the fuel canister with seemingly full flow from the LP, to a low pressure condition where I believe the injection pump is starving for fuel. I am going back out this morning and try to replicate last night's conditions and if possible watch real time readings of pressure from the fuel canister and electric current from the OPS while I am driving.

Please help if anyone has another idea or believes I missed something!