From my perspective, I noticed a couple things;
I doubt they are non-stock (or non-truck) injectors, unless the wiring and everything else was changed too ...it would have the fatty MultecII injectors. Unless by some chance they were swapped with 8.1 injectors, there really aren't any other stock looking injectors that would have ended up in there. If its got the fuel rail raised up and the tall skinny injectors, then ya we have something to address right away.
Tank pressure doesn't really do much for the engine, the only time it could set a code is for the emissions (i.e gas cap). You can suck gas out of a dish to bench run a motor.
Doobey
is also correct in looking at the FPR. You can unplug the vac source and see what psi its running. Its possible that a bad diaphram or stuck fpr could be causing some problems....its technically vacuum referenced so if there were to be a problem with it (or its vacuum source), you could lose fuel pressure.
Is this shop taking logs of any of this? With HPT looking right into the tune, things should stand out as apparent issue. And how the heck are we not getting any codes?
How is your fuel pump wired? Are you getting full voltage to the pump at all times? A bad ground, small gauge wire, or badly wired relay can all result in voltage drops as the pump, especially under load. Where is your fuel pump (in tank or inline), and what kind of filters are you running? W
Whatever the cause, I'm guessing the engine is either detecting, or getting a false positive for a lean condition. This causes the demand for more fuel, which the ECM tries to deliver, but just dumps 50% on top and neuters itself. While its possible there is still a problem in the tune, none of the "normal" problems do what you describe.