Yesterday pulled the new fuel filter off, blew it out, nothing inside. Blew the supply line to the fuel tank out, no obstruction at all.

Test drive: still doing it.

Today tried a new ECU: still doing it.
Put a new fuel pump on: still doing it.
Covered the intake and all vacuum sources with ether: slight rpm change at the EGR valve. Had the EGR unplugged anyway, but pulled the valve off and made a block-off for it.

Still doing it.

Base timing is at 14 degrees, emission label calls for 12. Truck starts perfect so no changes to timing made. Tested with and without vacuum advance and everything is working like it should.

Put the old carb back on since there is nothing wrong with it.

Still coughing and sputtering.

Any more ideas?