Is your unit equipped with an engine safety shut down system?
(two round relays and a small timer module). you said you changed ignitor but didn't mention coil or pick up. since it's impco it's probably not fuel related. what i would do is jump the distributor (black w/orange or black w/red) in a safe and interruptable way and drive the **** out of it. if it doesnt die it's an ignition voltage interuption. if it does it's in the distributor (probably)