One critical item that the PCM does not monitor is fuel pressure. Even though you have discounted the "
fuel injector, fuel pump, EGR valve, fuel filter, because we have had all of those things checked or replaced in the 5 years" the possibility exists that fuel pressure is the root of the problem. Your CPFI system requires a minimum or 61 PSIG fuel pressure to even start. I've seen a few start as low as 58, but not consistenly. However, once started, standing fuel pressure normally drops to 56-58 PSIG due to a vacuum signal being applied to the pressure regulator. I've seen some still run with pressure as low as 48 PSIG once started, although not very well.
One of the commonly overlooked problems with electric fuel pumps is the electrical connections. Pumps are very sensitive to voltage. On Astros with CPFI, there are four separate connectors in the pump circuit AFTER the fuel pump relay. This doesn't include teh relay itself and connections ahead of it. Any or all of these can be suspect. There is one connector just above and forward of the fuel tank. In addition to this, there is a ground connection in the same location that could cause poor connection. There is another connector at the top of the in-tank pump/sensor unit. There is another connector just inside the top of the in-tank unit, then there is a connector right at the pump itself.
This example was from an Astro that was actually running properly, yet still shows signs of poor connection and resistance:
Further pressure problems can be caused by the fluid connection between the pump and in-tank unit outlet. Some models incorporated a pulsation damper in this part of the system, and they can leak after time. There was a TSB issued by GM (66-63-09) for this problem. This should have been checked and the pulsation damper eliminated by the technician when the replacement pump was installed.
And if you have replaced the fuel filter, but five years ago, it's past time to do it again.