Car is surging slightly at low speeds

Occasionally acts as if out of time; coughs. When idling in gear going down the street, surges.

5.8 liter / automatic
E-250; heavy duty suspension and drive train
New repairs to date:
Radiator and heater core
New plugs, wires, dist. cap and rotor
Replace fuel filter and air filter
Replaced both exhaust manifolds due to cracking
Have not checked coil strength or fuel pump pressure