If it's weak at low rpms, and has an idle stumble, and this gets better with the vac removed from the FPR, it is responding positively to more fuel, since the purpose of that vac line is to hold a constant fuel pressure vs. manifold vacuum, an if it references atmo pressure instead, the fuel pressure is relatively higher, thus it injects more fuel.

This makes me think of vac leaks as the culprit, but those are usually masked at higher rpms as the relatively small amount of leakage (if it's huge at idle, it won't idle) becomes percentage wise of the total air flow a less significant factor. I've had lots of single runner intake leaks, and you lose the leaking manifold path cylinder(s) at idle, and they come back as you raise the rpms. The worst offender was a Toy LC I6 F engine, where the alloy intake mani always warped, AND was hard bolted to the cast iron exh mani.

So I think you have some kind of combo problem, like an intermittent connection maybe in some of the fi wiring to injectors and/or sensors, OR some kind of fuel volume flow problem, like a gummy intake sock on the fuel pump in the tank, a pump that is too internally worn to give the required flow as the R's come up, a fuel filter that got quickly gummed by stale fuel (I've done that to myself), a kinked fuel line, or a you name it in the fuel system.

I've clogged injector intake screens, jacked the truck up with a fuel line pinched, had a rubber line start to lose the inner liner and have a loose flap restrict only higher flow rates.

I'd like to see the flow vs. time test. Remove the fat wire from the starter and splice a hose onto the rubber line feeding high pressure fuel to the fuel rail, over under the TB. Have the son activate the ign sw. in the start position for say, 15 seconds, and measure the volume pumped. I'll SWAG an estimate of max fuel required at WOT at 5500rpms, which should be the max required. I think I can find the calcs for fuel vs. hp, and I'll use 150hp at that rpm to swag it.

I have a growing notion that gasahol gums worse when it sits for a while.

Might try a dose of Seafoam...

I've routinely seen 23-2400rpms of stall speed in the converter, and about 200-300rpms of TC slip at 3000 cruise without a lockup converter. You want the lockup if you can find it...