Don has a great idea. That test will bypass the ignition switch and ballast resistor and should let the truck continue to run if the problem is due to electrical issues. And it sounds like it is.
Maybe the old ignition switch is breaking down. Does it have a ballast resistor between the ignition switch and the coil?
You said you put a new coil on. Does it happen to have a built in resistor? It shouldn't have a built in resistor if you have a ballast resistor in the line from the ignition switch to the + side of the coil.
