Sputtering at Slow MPH/RPM

1975 Ford F250 w/390 high miles. Every now and again, and becoming more frequent with time, vehicle lurches at slow speeds. When clutch pushed in, engine revves fine and sounds great. Idling is usually poor, but not always. Sometimes high idle, sometimes low then dies. Always starts right back up. Fuel in bowl. Have replaced plugs/wires/rotor/cap/inline fuel filter, filter inside pump, accelerator pump diaphragm, ignition module. Old plugs were clean and obviously burning just right. Cap. At each contact was burnt and plastic badly melted. Strange though, replaced cap and did not make a difference. 8 Volts at coil when engine and key off. Ruled out air filter. Engine old but strong. Thinking electrical rather than fuel, but ACTS like fuel. Checked vacuum leaks, found a couple, but didn't resolve issue. Seems like having hard time when load applied to engine (idling, then turning steering or just driving at low speeds). Runs perfect at 40 MPH or higher. No oil/gas/water burn. Reading blogs on timing chain. Don't know where else to troubleshoot. What are your thoughts? Thank you kindly.