I have had these same symptoms before on different cars. The cause was different too.
FrankenDuster started great, idled great and would rev easily....UNTIL I put it in gear and leaned on it. THEN it would sputter, spit and stall out. That car had contaminated fuel. I drained the tank-8 gallons. Mixed in with the gas was a liquid that looked like Italian dressing. There were 2 quarts of it in there. I blew out the fuel line, let the tank dry out and put it all back together with a new filter. All better now.
The other rig was a 78 Dodge D 100 truck with a slant sick. It TOO showed exactly the same symptoms, but it turned out to have bad spark plug wires. They simply failed to deliver enough spark under a load.