When I'm out of ideas and can't find a solution, I stop, go back to the basics and start again from there.
First things first, do a FULL tune up on the vehicle, especially if you haven't done this yet. Obviously, you can just check the stuff you've already replaced, but if it hasn't been replaced by you in the last year or so, then do so now!
Plugs
Wires
Cap & rotor
Oil and filter change
Air filter change
Reset the ecu (unplug ecu for at least 10-20 minutes, longer is better)
Reset timing according to manual (don't forget short connector)
Check valve clearance
That's the full maintenance checklist. Now, time to do some investigating again. Do a quick check over all the vacuum lines, make sure there's no leaks or disconnected hoses. Test the TPS as per the manual. Test the afm also, make sure everything is working correctly there too. If you have one, do a compression or leak down test with your tester, always good to see if all is okay with the cylinders and valves closing properly.
When you pulled the plugs last, what did they tell you? White? Black? Sooty? Oily? Wet? Broken? A plug can tell you so many things about how the engine is running.
Now you can get down to the issue at-hand. You've already replaced most of the fuel delivery system, but you used some non-oem parts, so now its time to question whether those are actually good or not. Don't believe new parts go bad? Read up on some of this forum, you'll find a good deal of people who have used parts from AutoZone and other stores that didn't work or failed within days, but putting an oem part back in worked perfectly! And don't think the oem stuff won't fail either! They do, just not usual for this to happen.
Start from one end and test it all the way through. Fuel pump. Fuel lines. Fuel filter. Fuel rail. Injectors. Regulator. Return line. Charcoal canister.
Now, hit the electrical stuff. Again, injectors. Resistor pack. Fuses (I like to give 'em a good flick or light whack with a screwdriver handle, make sure they are solid and not intermittently shorting or breaking connection). Sensors (water temp, o2, oil pressure, all of 'em since you're there testing anyways).
It has to be somewhere in this whole list! There's really nothing left, aside from a failing ecu, which normally either works or it doesn't, but don't rule it out either.
You initially said this never happened until you put a load on the engine, then it just cut out. That still tells me that its gotta be electrical, usually sputtering will be mechanical, like fuel isn't being delivered constantly and at a regulated flow, if it was an on/off type of thing, then I still believe its electrical somewhere. Poke and prod around all the wiring while the engine is running. Yanking on all the wire looms (lightly of course!) may turn up a short or broken wire.
Almost all of this stuff is free to do, and I always say do the free stuff first, chances are you'll find the issue before you spend any money on unnecessary parts. come back and report your findings if something's not right. The guys here on t4r.org are pretty damned smart and they have fixed almost every person's dilemma, most of the time within hours!
