The point there was that if the relay pulls it's switching power right off the same wire that goes to the coil it could very well do what you are saying since your coil power supply seems to be about 6-7 volts. (which is a volt or two low)
Then when the relay activates the power to the coil dies down even farther.
Also a 7-800 RPM idle would be about normal, and if yours is below that the roundback alternator isn't putting out enough to maintain enough voltage to fire the coil sufficiently (especially after going through a ballast resistor)
My comment about the roundback alternator was because they have a very low charging output at idle (unlike the squareback style)
In your specific case I would disconnect that wire from the ignition and use the accessory circuit instead.
This can be found at the fuse block and is only powered with the key on, so it will all work the same. (except the relay won't be pulling power from the coil)
There are usually extra spades on the back of the fuse block where you can add the relay activation wire.
Betcha it works.:D
Sorry for further complicating this Rob, Rob and Del. :D