The vehicle: 1984 Ram 318 4 speed.

Issue: In the top gears at low rpm (above idle to 1800 or so) if I put it down to 3/4 throttle the truck starts sputtering and wanting to quit like its' out of fuel. From a standing start I can hold it to the floor and row all the way through the gears, rev it to the moon, no problem. It has power to burn and the fuel mileage is very good. When I bought the truck a few weeks ago it had no such issue at all pulling itself at low rpm.

What has been done so far:

New fuel filter with the line to the tank blown out. All clear.
New fuel pump.
Checked needle and seat, no restrictions.
New cap, rotor, wires, and plugs. None of the plug wires are crossed.
New ECU.
New coil.
Checked vacuum advance and it is working properly.
Base timing calls for 12 degrees. It is set on 14 and starts with just a tap of the key. I moved the timing up to 16-no change, moved it down to 12-no change. Full mechanical advance is in the 32 range. Tried driving with the advance unhooked-no change.
The idle is rock steady at 750 with 19 inches vacuum and throttle response is good.
EGR valve is unhooked, but I took it apart anyway and made a block-off plate.
There is no catalytic converter.
PCV valve works.
The carb is a Holley 2280 with some emission stuff on it. I wired the choke open and drove it and it still coughs and sputters. Have a perfect working 2280 on my '71 so I pulled it off and put it on the truck. Amazingly, same thing, still coughing and sputtering.
I have covered the whole intake manifold, lines, and brake booster with ether and found no noticible rpm change.

So there you have it. I have tried everything I can think of over the last few weeks, replaced a bunch of parts and still have the same problem. It's getting worse.

Does anyone know what it could be?