I ran into a simular problem. My truck would run good around a couple of blocks. I would take it out again or after running it for an extended time it would slowly lose power sputter and die and would not restart. After sitting for about 20 min it would start for a little while.

I noticed that my points were burnt. After reading some posts on 12 volt conversions there was a component that the coil needed. A ballast resistor. Some coils have an internal resistor, those that don't usually say on the body of the coil. If the coil says "Use with external resistor" then an external ballast resistor is needed. You see these on the firewall of some vehicles. They can be bought at NAPA or other places for less than $10. After replacing the 12 volt coil, installing a ballast and installing a 12 volt condenser the problem was solved.

Check your points if you have them and see if they are discolored. Hope this narrows it down.

Rick...........