I Have a 1951 Ford COE on a 76 GM 1 tom dually with a 76 caddy 500 cu in big block. It has a HEI distributor, external mech fuel pump and filter. No eng problems until a few weeks ago whern it started to occasionally misfire at speed. Problem seems to worsen has the engine reaches temp. Idles OK cold and warm. Have checked timing ok at 4 BTDC steady and steady advance with increase of RPM. Engine vac ~17" and steady. Have replaced plugs, points, coil, cap, rotor, fuel filter and pump. Have installed voltmeter on hei coil voltage lead to check for steady supply during next road test. Getting very fustrated on the cause. Any suggestions welcome! Thanks, Johnny52